compositor-worker: Remove code from cc_blink

In this CL, the compositor worker code in cc_blink has
been removed, mostly by moving it into
Source/platform/graphics, a more natural home for
compositor-related stuff.

An additional benefit is that much of the code in
public/platform/ is unnecessary now. It was there only
so that it could be implemented by classes in cc_blink.

There was a wrinkle when moving the test: the cc test
machinery expected a base::MessageLoop to have been
constructed to work. This violated the current DEPS
rules for platform/graphics.

I did try to remove this dependency, but the code is
quite coupled and the refactor was not only large, it
resulted in more complex code in cc.

Since platform is in general allowed to talk to base,
it seemed more reasonable to update the DEPS in
platform/graphics/ to allow the use of message_loop.h
in tests.

BUG=430155
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1599673002

Cr-Commit-Position: refs/heads/master@{#371006}
39 files changed