Handle MemoryPressureListenerRegistry in WorkerThread

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

The first step consists in moving MemoryPressureListenerRegistry's
RegisterThread/UnregisterThread() to WorkerThread. In order to do that
we have to add a new thread creation parameter to decide whether or not
the new thread will support garbage collection or not.

Bug: 961743
Change-Id: I662cd31f24f8dc2d47697ff026251b80b31cbe32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611985
Commit-Queue: Sergio Villar <svillar@igalia.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661333}
6 files changed