Use SimpleThread and SimpleThreadScheduler in PpapiBlinkPlatform.

This CL lets PpapiThread use CreateMainThreadAndInitialize() so the
Platform class can create the main thread on behalf.

Previously, the code does not create an WebThread instance for the main
thread(!), and PpapiBlinkPlatform::CurrentThread() actually returns
nullptr on the main thread(!). This is because BlinkPlatformImpl::
CurrentThread() only cares about non-main threads created with
CreateThread(). This CL fixes this issue.

Bug: 826203
Change-Id: Icc1319e70da2d7c2a3ccaee0adb140df80cd496d
Reviewed-on: https://chromium-review.googlesource.com/1186019
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590188}
7 files changed