Intermittent crash ("failed to fill whole buffer") in /html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_nonescaping-1.html, /navigation-timing/nav2_test_open_data_uri.html and /_mozilla/css/iframe/multiple_external.html

[jdm]

Continuation of #14323 because the title became too long for github.

  ▶ CRASH [expected TIMEOUT] /html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_nonescaping-1.html
  │ 
  │ VMware, Inc.
  │ Gallium 0.4 on softpipe
  │ 3.3 (Core Profile) Mesa 12.0.1
  │ ERROR:script::dom::bindings::error: Error at http://web-platform.test:8000/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_popups_helper-1.html:3:1 opener is not defined
  │ called `Result::unwrap()` on an `Err` value: Error { repr: Custom(Custom { kind: UnexpectedEof, error: StringError("failed to fill whole buffer") }) } (thread RenderBackend, at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libcore/result.rs:870)
  │ stack backtrace:
  │    0:     0x7efc840ef12d - backtrace::backtrace::trace::h3daaf829744ddab9
  │    1:     0x7efc840ef822 - backtrace::capture::Backtrace::new::hd6fb6e83f9059b0d
  │    2:     0x7efc82bb6737 - servo::main::{{closure}}::h2a03f49e668fc733
  │    3:     0x7efc84a4c538 - std::panicking::rust_panic_with_hook
  │                         at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/panicking.rs:556
  │    4:     0x7efc84a4c404 - std::panicking::begin_panic<collections::string::String>
  │                         at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/panicking.rs:517
  │    5:     0x7efc84a4c329 - std::panicking::begin_panic_fmt
  │                         at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/panicking.rs:501
  │    6:     0x7efc84a4c2b7 - std::panicking::rust_begin_panic
  │                         at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/panicking.rs:477
  │    7:     0x7efc84a7992d - core::panicking::panic_fmt
  │                         at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libcore/panicking.rs:69
  │    8:     0x7efc8413d6ad - core::result::unwrap_failed::h2d1ca41324c527be
  │    9:     0x7efc84182530 - webrender::render_backend::RenderBackend::run::h0a19d480cc9fee1e
  │   10:     0x7efc8412d33d - std::panicking::try::do_call::h0a48dab7951a70dd
  │   11:     0x7efc84a5347a - panic_unwind::__rust_maybe_catch_panic
  │                         at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libpanic_unwind/lib.rs:98
  │   12:     0x7efc84147ac7 - <F as alloc::boxed::FnBox<A>>::call_box::hf8ea3309c4c01e4d
  │   13:     0x7efc84a4b1c4 - alloc::boxed::{{impl}}::call_once<(),()>
  │                         at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/liballoc/boxed.rs:615
  │                          - std::sys_common::thread::start_thread
  │                         at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/sys_common/thread.rs:21
  │                          - std::sys::imp::thread::{{impl}}::new::thread_start
  │                         at /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libstd/sys/unix/thread.rs:84
  │   14:     0x7efc80a82183 - start_thread
  │   15:     0x7efc8059937c - clone
  │   16:                0x0 - <unknown>
  │ ERROR:servo: called `Result::unwrap()` on an `Err` value: Error { repr: Custom(Custom { kind: UnexpectedEof, error: StringError("failed to fill whole buffer") }) }
  └ Pipeline failed in hard-fail mode.  Crashing!

[jdm]

Given the decision to remove the filenames from #14323 we don't need this any longer.