This only affects debug builds, so it's not a P0.

This looks like a problem either in the compiler or PNaCl translator.
Problem is that SequencedTaskRunnerHandle::IsSet() returns true, when it should return false.

bool SequencedTaskRunnerHandle::IsSet() {
  return lazy_tls_ptr.Pointer()->Get() ||
         SequencedWorkerPool::GetSequenceTokenForCurrentThread().IsValid() ||
         base::ThreadTaskRunnerHandle::IsSet();
}

When this code is executed each of these 3 conditions is false, but the function returns true.
Still investigating what the root cause is.

The PNaCl code generated by the compiler seems correct, so it looks like the problem is in the PNaCl translator. 
Just reordering the operands in the return statement makes issue go away:

  return SequencedWorkerPool::GetSequenceTokenForCurrentThread().IsValid() ||
      base::ThreadTaskRunnerHandle::IsSet() || lazy_tls_ptr.Pointer()->Get();

Actually it appears the the issue is caused by optimizer in the PNaCl linker. The .o file contains correct code, but the optimized code in pexe is incorrect. I see the following code at the end of SequencedTaskRunnerHandle::IsSet() in the pexe file:

cleanup.done:                                     ; preds = %lor.end.cleanup.done_crit_edge, %_ZN4base19SequencedWorkerPool32GetSequenceTokenForCurrentThreadEv.exit, %_ZN4base18ThreadLocalPointerINS_25SequencedTaskRunnerHandleEE3GetEv.exit
  %.ret_ext = zext i1 true to i32, !dbg !1124856
  ret i32 %.ret_ext, !dbg !1124856

I.e. the function always returns true. This is obviously wrong.

The problem also goes away when I add __attribute__((optnone)) to SequencedTaskRunnerHandle::IsSet(), which disables all optimizations for that function.

+bradnelson@ for triage.
I'm going to land https://codereview.chromium.org/2384553002/ as a workaround for CRD.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f1f0af26d41c5119403b0781d804f7860e237871

commit f1f0af26d41c5119403b0781d804f7860e237871
Author: sergeyu <sergeyu@chromium.org>
Date: Thu Sep 29 21:27:03 2016

Disable link-time optimization for remoting PNaCl plugin in Debug mode

Optimization was enabled for the plugin to workaround an issue with
linking libyuv. But that issue has been fixed, so it's not longer
necessary to optimize debug builds.

This also works around PNaCl link-time optimization bug that was
breaking debug builds. That bug currently doesn't affect non-debug
builds (if it breaks release builds in the future it's also possible
to work it around by disabling optimization with per-function
optnone attribute).

BUG=650499

Review-Url: https://codereview.chromium.org/2384553002
Cr-Commit-Position: refs/heads/master@{#421938}

[modify] https://crrev.com/f1f0af26d41c5119403b0781d804f7860e237871/remoting/client/plugin/BUILD.gn

Just stumbled on this, fixed?

Assigning to bradnelson@chromium.org for triage. Maybe this is fixed now with the revert?

ping?

Please close if this is no longer an issue (and it definitely isn't a TaskScheduler implementation issue so I'm removing that label).

Thanks!

This bug has an owner, thus, it's been triaged. Changing status to "assigned".