GC hazard during window proxy transplanting

[jdm]

Running with very zealous GC uncovered this panic:

26:24.73 pid:99336 Full command: /Users/worker/tasks/task_1567196401/repo/target/release/servo --hard-fail -u Servo/wptrunner -Z replace-surrogates -z http://web-platform.test:8000/xhr/open-url-about-blank-window.htm --user-stylesheet /Users/worker/tasks/task_1567196401/repo/resources/ahem.css --certificate-path /Users/worker/tasks/task_1567196401/repo/tests/wpt/web-platform-tests/tools/certs/cacert.pem --pref=js.mem.gc.zeal.level=2 --pref=js.mem.gc.zeal.frequency=10
pid:99336 
27:54.58 pid:99336 assertion failed: self.is_double() (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(1) }, at /Users/worker/.cargo/git/checkouts/mozjs-fa11ffc7d4f1cc2d/2bf4912/src/jsval.rs:439)
27:55.22 pid:99336 stack backtrace:
27:55.22 pid:99336    0: __ZN9backtrace9backtrace5trace17hc4a0d1f273e07024E (0x112fdf23b)
27:55.22 pid:99336    1: __ZN9backtrace7capture9Backtrace3new17h765760e39b281a50E (0x112fde427)
27:55.22 pid:99336    2: __ZN5servo4main28_$u7b$$u7b$closure$u7d$$u7d$17h510947304905906eE (0x10f74193b)
27:55.22 pid:99336    3: __ZN3std9panicking20rust_panic_with_hook17h099fe3773ce1a793E (0x11316b301)
27:55.22 pid:99336    4: __ZN3std9panicking11begin_panic17h58d48b534ada9bb5E (0x10fcb3ec4)
27:55.22 pid:99336    5: __ZN6script3dom11windowproxy5trace17h3f158da072087e7cE (0x11041ef63)
27:55.22 pid:99336    6: __ZNK19WrapperProxyHandler5traceEP8JSTracerP8JSObject (0x110aae72e)
27:55.22 pid:99336    7: __ZN2js5Proxy5traceEP8JSTracerP8JSObject (0x1111b20a2)
27:55.22 pid:99336    8: __ZN2js11ProxyObject5traceEP8JSTracerP8JSObject (0x1111b238f)
27:55.23 pid:99336    9: __ZNK2js5Class7doTraceEP8JSTracerP8JSObject (0x11113a09b)
27:55.23 pid:99336   10: __ZL13CallTraceHookIZN2js8GCMarker19processMarkStackTopERNS0_11SliceBudgetEEUlT_E_EPNS0_12NativeObjectEOS4_P8JSTracerP8JSObject15CheckGeneration (0x1113caa43)
27:55.23 pid:99336   11: __ZN2js8GCMarker19processMarkStackTopERNS_11SliceBudgetE (0x11138338f)
27:55.23 pid:99336   12: __ZN2js8GCMarker24markUntilBudgetExhaustedERNS_11SliceBudgetE (0x1113675f4)
27:55.23 pid:99336   13: __ZN2js2gc9GCRuntime24markUntilBudgetExhaustedERNS_11SliceBudgetENS_7gcstats9PhaseKindE (0x1113643a5)
27:55.23 pid:99336   14: __ZN2js2gc9GCRuntime16incrementalSliceERNS_11SliceBudgetEN2JS8GCReasonERNS0_13AutoGCSessionE (0x11136c623)
27:55.23 pid:99336   15: __ZN2js2gc9GCRuntime7gcCycleEbNS_11SliceBudgetEN2JS8GCReasonE (0x11136e2f2)
27:55.23 pid:99336   16: __ZN2js2gc9GCRuntime7collectEbNS_11SliceBudgetEN2JS8GCReasonE (0x11136f9a3)
27:55.23 pid:99336   17: __ZN2js2gc9GCRuntime2gcE18JSGCInvocationKindN2JS8GCReasonE (0x111339176)
27:55.23 pid:99336   18: __ZN2js2gc9GCRuntime10runDebugGCEv (0x111338ef4)
27:55.23 pid:99336   19: __ZN2js2gc9GCRuntime22gcIfNeededAtAllocationEP9JSContext (0x111338865)
27:55.23 pid:99336   20: __ZN2js2gc9GCRuntime19checkAllocatorStateILNS_7AllowGCE1EEEbP9JSContextNS0_9AllocKindE (0x111333eb1)
27:55.23 pid:99336   21: __ZN2js8AllocateINS_11ObjectGroupELNS_7AllowGCE1EEEPT_P9JSContext (0x111336dd8)
27:55.23 pid:99336   22: __ZN2js16ObjectGroupRealm9makeGroupEP9JSContextPN2JS5RealmEPKNS_5ClassENS3_6HandleINS_11TaggedProtoEEEj (0x110df1b48)
27:55.23 pid:99336   23: __ZN8JSObject13makeLazyGroupEP9JSContextN2JS6HandleIPS_EE (0x110df192b)
27:55.23 pid:99336   24: __ZN8JSObject8getGroupEP9JSContextN2JS6HandleIPS_EE (0x110d6180e)
27:55.23 pid:99336   25: __ZN8JSObject4swapEP9JSContextN2JS6HandleIPS_EES5_ (0x110d60263)
27:55.23 pid:99336   26: __Z19JS_TransplantObjectP9JSContextN2JS6HandleIP8JSObjectEES5_ (0x111109421)
27:55.24 pid:99336   27: __ZN6script3dom11windowproxy11WindowProxy10set_window17hd0bbc9e139d65ed3E.llvm.1967242945246974025 (0x11041d1c1)
27:55.24 pid:99336   28: __ZN6script13script_thread12ScriptThread18local_window_proxy17h15a1c04525d96e43E (0x11043d838)
27:55.24 pid:99336   29: __ZN6script13script_thread12ScriptThread4load17h3bc1fbe7198f3867E (0x11043eb7f)
27:55.24 pid:99336   30: __ZN6script13script_thread12ScriptThread29handle_page_headers_available17hb22af78b56e10e56E (0x11043b5c9)
27:55.24 pid:99336   31: __ZN3std6thread5local17LocalKey$LT$T$GT$4with17hc55422f5ce60603dE (0x1102dfd52)
27:55.24 pid:99336   32: __ZN93_$LT$script..dom..servoparser..ParserContext$u20$as$u20$net_traits..FetchResponseListener$GT$16process_response17hb3a512286bee2689E (0x10ff7a9b5)
27:55.24 pid:99336   33: __ZN6script13script_thread12ScriptThread17handle_new_layout17h7de5adf3a95a1586E (0x11043a560)
27:55.24 pid:99336   34: __ZN6script13script_thread12ScriptThread11handle_msgs17h689a61696977da24E.llvm.1967242945246974025 (0x1104294d4)
27:55.24 pid:99336   35: __ZN14profile_traits3mem12ProfilerChan25run_with_memory_reporting17h7b6e9bd38e1ab8feE (0x11055eea7)
27:55.24 pid:99336   36: __ZN3std10sys_common9backtrace28__rust_begin_short_backtrace17he46720d101a3e6efE (0x11080c47b)
27:55.24 pid:99336   37: __ZN3std9panicking3try7do_call17h5f1d988e7dc73db4E.llvm.16932765531113996662 (0x10fcd96aa)
27:55.24 pid:99336   38: ___rust_maybe_catch_panic (0x11317369e)
27:55.24 pid:99336   39: __ZN4core3ops8function6FnOnce40call_once$u7b$$u7b$vtable.shim$u7d$$u7d$17hc92a2538c3cac723E (0x10ff15bf5)
27:55.24 pid:99336   40: __ZN83_$LT$alloc..boxed..Box$LT$F$GT$$u20$as$u20$core..ops..function..FnOnce$LT$A$GT$$GT$9call_once17hb03304ace7f89c63E (0x11315aa2d)
27:55.24 pid:99336   41: __ZN3std3sys4unix6thread6Thread3new12thread_start17hd2d570f25bb299f3E (0x113172bcd)
27:55.24 pid:99336   42: __pthread_body (0x7fff63578660)
27:55.24 pid:99336   43: __pthread_start (0x7fff6357850c)
27:55.24 pid:99336 [2019-08-30T20:50:15Z ERROR servo] assertion failed: self.is_double()

[gterzian]

Also noted at #23959