Add a SharedMainThreadContextProvider method to Platform

This CL is a preparation step to Onion soup webmediaplayer_ms.cc|h
and webmediaplayer_ms_compositor.cc|h
It adds to blink::Platform a methods that expose SharedMainThreadContextProvider()
functionality of RenderThreadImpl needed by WebMediaPlayerMS.

It is part of the phase 4.4 in the design doc [1].

[1] https://docs.google.com/document/d/1rHJGi1U72qZsOGIctIf7GKRaY8d7BgtVW8MaMYumzYY/

BUG=704136
[email protected], [email protected]

Change-Id: I3e6534e1a7e362da905ddd78e7e88d51adb5e6eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1693241
Commit-Queue: Antonio Gomes (use @igalia instead) <[email protected]>
Reviewed-by: Kentaro Hara <[email protected]>
Reviewed-by: Guido Urdaneta <[email protected]>
Cr-Commit-Position: refs/heads/master@{#676201}
3 files changed