Probably we can work around this, by
- making WorkerEventQueue::EventDispatcherTask non-ExecutionContextTask, and
- using createSameThreadTask() with std::unique_ptr<WorkerEventQueue::EventDispatcherTask>.

Similar thing is done in https://codereview.chromium.org/1976083002.

tzik@ is working on this issue: https://codereview.chromium.org/2209613002/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3697042db3e1ee433545c06bed5aee95657ebc16

commit 3697042db3e1ee433545c06bed5aee95657ebc16
Author: tzik <tzik@chromium.org>
Date: Thu Aug 04 07:41:56 2016

Remove WorkerEventQueue::EventDispatcherTask

EventDispatcherTask is the last blocker of ExecutionContextTask removal.
It holds an Event and WorkerEventQueue instance, and makes the Event
cancellable.

This CL reimplements the cancellation mechanism without the custom
ExecutionContextTask and removes the custom one.

BUG= 630139 

Review-Url: https://codereview.chromium.org/2209613002
Cr-Commit-Position: refs/heads/master@{#409732}

[modify] https://crrev.com/3697042db3e1ee433545c06bed5aee95657ebc16/third_party/WebKit/Source/core/workers/WorkerEventQueue.cpp
[modify] https://crrev.com/3697042db3e1ee433545c06bed5aee95657ebc16/third_party/WebKit/Source/core/workers/WorkerEventQueue.h