Use Thread::Current()/MainThread() in core/.

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

Bug: 826203
Change-Id: I18a106751518e361aecfb6e82152359c1b00f3d1
Reviewed-on: https://chromium-review.googlesource.com/c/1319129
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605595}
23 files changed