[ThreadPool] Rename base::ThreadPool to base::ThreadPoolInstance

In order to leave the base::ThreadPool symbol available for
an upcoming "destination" task trait.

Also moved ThreadPoolImpl to be the implementation of TaskExecutor,
not ThreadPoolInstance. It was a mistake that base::ThreadPool was
implementing TaskExecutor, its users shouldn't have access to
PostTask*() and Create*TaskRunner*().

TBR=fdoray@chromium.org
(bypass owners for side-effects beyond //base)

Bug: 968047
Change-Id: I0607fba6d7f30d202bf7f61a9f461b1256e87467
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1634851
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664300}