Introduce a best-effort task queue responsible for renderer memory purges

Currently, the timer responsible for initiating renderer memory purges
uses the default task runner. Since memory purging is a proactive
optimization, we should not spend time cleaning when we have more
important work to do.

A new best-effort task queue was added to MainThreadSchedulerImpl to
give memory purges lower priority than more important tasks.

Bug: 926186
Change-Id: I06a31188ac5d84ee3846eabb4187d10285d562c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1525998
Commit-Queue: Aditya Keerthi <adityakeerthi@google.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643356}
10 files changed