Use Thread::Current()/MainThread()/CompositorThread() in platform/.

This CL simplifies lengthy function calls via Platform::Current() that
obtain the current, main or compositor thread by using Thread::
Current(), Thread::MainThread() or Thread::CompositorThread().

Bug: 826203
Change-Id: I791712e02848b8792204b598fc7fa19114ae36cd
Reviewed-on: https://chromium-review.googlesource.com/c/1319395
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606379}
20 files changed