Add preferences to enable/disable the JITs

When creating a runtime (script.rs), we check prefs and then enable or
disable the JITs (Baseline and Ion).

components/script/script_thread.rs

@@ -57,7 +57,7 @@ use js::jsapi::{DisableIncrementalGC, JS_AddExtraGCRootsTracer, JS_SetWrapObject
 use js::jsapi::{GCDescription, GCProgress, JSGCInvocationKind, SetGCSliceCallback};
 use js::jsapi::{JSAutoRequest, JSGCStatus, JS_GetRuntime, JS_SetGCCallback, SetDOMCallbacks};
 use js::jsapi::{JSContext, JSRuntime, JSTracer};
-use js::jsapi::{JSObject, SetPreserveWrapperCallback};
+use js::jsapi::{JSObject, RuntimeOptionsRef, SetPreserveWrapperCallback};
 use js::jsval::UndefinedValue;
 use js::rust::Runtime;
 use layout_interface::{ReflowQueryType};
@@ -108,6 +108,7 @@ use task_source::user_interaction::UserInteractionTaskSource;
 use time::{Tm, now};
 use url::Url;
 use util::opts;
+use util::prefs::get_pref;
 use util::str::DOMString;
 use util::thread;
 use util::thread_state;
@@ -739,6 +740,15 @@ impl ScriptThread {
             DisableIncrementalGC(runtime.rt());
         }
 
+        // Enable or disable the JITs.
+        let rt_opts = unsafe { &mut *RuntimeOptionsRef(runtime.rt()) };
+        if let Some(val) = get_pref("js.baseline.enabled").as_boolean() {
+            rt_opts.set_baseline_(val);
+        }
+        if let Some(val) = get_pref("js.ion.enabled").as_boolean() {
+            rt_opts.set_ion_(val);
+        }
+
         runtime
     }
 

resources/prefs.json

@@ -2,6 +2,8 @@
   "dom.mouseevent.which.enabled": false,
   "dom.mozbrowser.enabled": false,
   "gfx.webrender.enabled": false,
+  "js.baseline.enabled": true,
+  "js.ion.enabled": true,
   "layout.columns.enabled": false,
   "layout.column-width.enabled": false,
   "layout.column-count.enabled": false,