Add (NonMain)ThreadScheduler::Current().

The function returns the current thread's instance of ThreadScheduler.
This is an alias of the "Platform::Current()->CurrentThread()
->Scheduler()" idiom which is currently widely used in Blink, and all
such calls will be replaced with the shorter "ThreadScheduler
::Current()" call in the future.

This patch also adds similar NonMainThreadScheduler::Current(), which
DCHECKs the caller is on the right thread. When we add
MainThreadScheduler in the future (this is likely to happen), it will
also have the similar Current() function.

Bug: 826203
Change-Id: Iaba856ff37cb81a33cbdf4b558764ab5634adf84
Reviewed-on: https://chromium-review.googlesource.com/1065538
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559857}
5 files changed