The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/32dd9efba59837681df3260708f25024ede1fe98

commit 32dd9efba59837681df3260708f25024ede1fe98
Author: hongchan <hongchan@chromium.org>
Date: Tue Oct 11 21:35:09 2016

Refactor BackingThreadHolder to WorkletThreadHolder

This CL refactors BackingThreadHolder, which was a part of
AbstractAnimationWorkletThread, to core/workers/WorkletThreadHolder.
Because AudioWorkletThread and AnimationWorkletThread share this
backing thread holder, it is reasonable to factor this part out and
make it a part of core/worker.

BUG= 652750 
TEST=(The build passes AnimationWorkletThreadTest.)

Review-Url: https://codereview.chromium.org/2389193003
Cr-Commit-Position: refs/heads/master@{#424551}

[modify] https://crrev.com/32dd9efba59837681df3260708f25024ede1fe98/third_party/WebKit/Source/core/workers/BUILD.gn
[add] https://crrev.com/32dd9efba59837681df3260708f25024ede1fe98/third_party/WebKit/Source/core/workers/WorkletBackingThreadHolder.cpp
[add] https://crrev.com/32dd9efba59837681df3260708f25024ede1fe98/third_party/WebKit/Source/core/workers/WorkletBackingThreadHolder.h
[modify] https://crrev.com/32dd9efba59837681df3260708f25024ede1fe98/third_party/WebKit/Source/modules/compositorworker/AbstractAnimationWorkletThread.cpp