Remove WebThreadSupportingGC::Post*Task() methods

WebThreadSupportingGC is mostly wrapping blink::Thread. Code should use
blink::scheduler::WorkerThread instead.

This CL replaces all the WebThreadSupportingGC::Post*Task() methods by
TaskRunner's Post*Task() ones. That requires a new temporary API in
WebThreadSupportingGC to retrieve the TaskRunner. This will ease the transition
from WebThreadSupportingGC to WorkerThread.

Bug: 961743
Change-Id: I7f323e1a3d79b7c70ee45152df90b841b9a5975f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1625107
Commit-Queue: Sergio Villar <svillar@igalia.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662564}
14 files changed