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This CL allows base::ThreadChecker in //src/third_party/WebKit/Source and uses
it in WorkerThreadLifecycleContext as an example.

WorkerThreadLifecycleContext is now bound with the main thread and
DCHECK(IsMainThread) checks it, but the restriction will be removed by ongoing
CLs soon (see the associated issue). base::ThreadChecker should be useful in
such a case.

Bug: 829119
Change-Id: I01a2d8663829e6e2ad2d572664ff0a5ced14a4ae
Reviewed-on: https://chromium-review.googlesource.com/997232Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548383}