Use ThreadScheduler::Current() in platform/.

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

Also, this CL simplifies the code that assumes Platform::CurrentThread()
or Thread::Scheduler() can return null, which is no longer true.

Bug: 826203
Change-Id: I0a59ef17be978d3a4d8b229dc19069aeb0a4c2f1
Reviewed-on: https://chromium-review.googlesource.com/c/1314084
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@{#605275}
6 files changed