Use ThreadScheduler::Current() in core/.

This CL replaces the occurrences of "Platform::Current()
->CurrentThread()->Scheduler()" with "ThreadScheduler::Current()"
which has the same meaning.

Bug: 826203
Change-Id: I53ef4f8682620b217d8cb6158a3a3106ae75eb06
Reviewed-on: https://chromium-review.googlesource.com/c/1314080
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604876}
13 files changed