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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().

Also now-redundant null checks for those functions are removed.

Bug: 826203
Change-Id: Iedfd7f09b5334dc84fa921cdca84ca39ad9b3af9
Reviewed-on: https://chromium-review.googlesource.com/c/1318773Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605600}