After  issue 694914  TaskRunnerHelper becomes available for worker contexts, but it's still necessary to call it on a given context's thread ("given context" is the 2nd arg of TaskRunnerHelper::Get) as follows:

  // This must be called on the main thread
  TaskRunnerHelper::Get(type, frame)

  // This must be called on the main thread
  TaskRunnerHelper::Get(type, document)

  // This must be called on the worker thread
  TaskRunnerHelper::Get(type, worker_global_scope)

I'm now checking how many callsites violate the rule by adding DCHECKs:
https://codereview.chromium.org/2829683006/

hajimehoshi@ is now working on refactoring TaskRunnerHelper.

>   TaskRunnerHelper::Get(type, worker_global_scope)

I think it's now Get(TaskType, WorkerOrWorkletGlobalScope*), and this already has DCHECK(IsContextThread());

Fix for Document
https://chromium-review.googlesource.com/c/chromium/src/+/741428

Note that we found that |Context()->GetExecutionContext()| is also not thread-safe (SuspendableObject::GetExecutionContext should be called from the main thread).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0d0c1a51c94302df6ddaef1df81bce83ef520077

commit 0d0c1a51c94302df6ddaef1df81bce83ef520077
Author: Hajime Hoshi <hajimehoshi@chromium.org>
Date: Fri Oct 27 13:39:37 2017

Add DCHECK(IsMainThread()) at Document::GetTaskRunner()

Bug:  696905 
Change-Id: Ie6f81336a9119f0cd8d3e3ad6e9074a11f662b8d
Reviewed-on: https://chromium-review.googlesource.com/741428
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512170}
[modify] https://crrev.com/0d0c1a51c94302df6ddaef1df81bce83ef520077/third_party/WebKit/Source/core/dom/Document.cpp
[modify] https://crrev.com/0d0c1a51c94302df6ddaef1df81bce83ef520077/third_party/WebKit/Source/modules/webaudio/AudioScheduledSourceNode.cpp
[modify] https://crrev.com/0d0c1a51c94302df6ddaef1df81bce83ef520077/third_party/WebKit/Source/modules/webaudio/AudioScheduledSourceNode.h
[modify] https://crrev.com/0d0c1a51c94302df6ddaef1df81bce83ef520077/third_party/WebKit/Source/modules/webaudio/MediaElementAudioSourceNode.cpp
[modify] https://crrev.com/0d0c1a51c94302df6ddaef1df81bce83ef520077/third_party/WebKit/Source/modules/webaudio/MediaElementAudioSourceNode.h
[modify] https://crrev.com/0d0c1a51c94302df6ddaef1df81bce83ef520077/third_party/WebKit/Source/modules/webaudio/OfflineAudioDestinationNode.cpp
[modify] https://crrev.com/0d0c1a51c94302df6ddaef1df81bce83ef520077/third_party/WebKit/Source/modules/webaudio/OfflineAudioDestinationNode.h
[modify] https://crrev.com/0d0c1a51c94302df6ddaef1df81bce83ef520077/third_party/WebKit/Source/modules/webaudio/ScriptProcessorNode.cpp
[modify] https://crrev.com/0d0c1a51c94302df6ddaef1df81bce83ef520077/third_party/WebKit/Source/modules/webaudio/ScriptProcessorNode.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3e1e063396152e6992d6aa114e604bfc10f296fb

commit 3e1e063396152e6992d6aa114e604bfc10f296fb
Author: Hajime Hoshi <hajimehoshi@chromium.org>
Date: Mon Oct 30 08:25:10 2017

Add DCHECK(IsMainThread()) to LocalFrame::GetTaskRunner()

Bug:  696905 
Change-Id: I12008dc1431d370e45df3fcd6d38184457133bd9
Reviewed-on: https://chromium-review.googlesource.com/741661
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512455}
[modify] https://crrev.com/3e1e063396152e6992d6aa114e604bfc10f296fb/third_party/WebKit/Source/core/frame/LocalFrame.cpp

> Note that we found that |Context()->GetExecutionContext()| is also not thread-safe (SuspendableObject::GetExecutionContext should be called from the main thread).

It'd be nice if we can have a thread check in GetExecutionContext(). IMHO, ExecutionContext shouldn't be touched from threads other than its context thread.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a989f82576b212a93932904916dbc2c103dc071a

commit a989f82576b212a93932904916dbc2c103dc071a
Author: Hajime Hoshi <hajimehoshi@chromium.org>
Date: Tue Oct 31 17:01:28 2017

Avoid calling Context()->GetExecutionContext() on non-main threads in webaudio

Checking the existence of execution context is not thread safe. Instead,
let's check it on the handler sides (main thread).

Bug:  696905 
Change-Id: I68a05ef9e32837f74ef1772d66631bfd104c7d5e
Reviewed-on: https://chromium-review.googlesource.com/746721
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Reviewed-by: Raymond Toy <rtoy@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512850}
[modify] https://crrev.com/a989f82576b212a93932904916dbc2c103dc071a/third_party/WebKit/Source/modules/webaudio/AudioScheduledSourceNode.cpp
[modify] https://crrev.com/a989f82576b212a93932904916dbc2c103dc071a/third_party/WebKit/Source/modules/webaudio/MediaElementAudioSourceNode.cpp
[modify] https://crrev.com/a989f82576b212a93932904916dbc2c103dc071a/third_party/WebKit/Source/modules/webaudio/OfflineAudioDestinationNode.cpp
[modify] https://crrev.com/a989f82576b212a93932904916dbc2c103dc071a/third_party/WebKit/Source/modules/webaudio/ScriptProcessorNode.cpp

> It'd be nice if we can have a thread check in GetExecutionContext(). IMHO, ExecutionContext shouldn't be touched from threads other than its context thread.

True, I'll add DCHECK later. I'll close this issue as the focused problem was fixed.