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This is a prerequisite to enable ScopedTaskEnvironment MOCK_TIME
on top of any RunLoop::Delegate (i.e. on top of MessageLoopForUI/ForIO).

This CL also removes RunLoop::Delegate::Client::IsNested() as it
was a mere shortcut for its TLS complement and had to switch to
using TLS itself to remain valid in override scenarios...
Ran base_perftests.exe --gtest_filter=*MessageLoop* in static/Release
and things look the same.

Bug: 708584
Change-Id: I143f6e6afb47de11f95702c337dbe63eb0887596
Reviewed-on: https://chromium-review.googlesource.com/784214
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521228}