CacheStorage: Schedule operations using an explicit TaskRunner.

Currently the CacheStorageScheduler use ThreadTaskRunnerHandler::Get()
to get the TaskRunner used to schedule operations.  In crbug.com/960012
we want to experiment with running the cache_storage code on a
SequencedTaskRunner instead of the IO thread.  As a first step this CL
refactors the code so we can pass a specific SequencedTaskRunner down
to the scheduler.

Bug: 960012
Change-Id: Icdb52a01ca4eee9afe8b30f5f04cf1ef04c1eaa7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1599304
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659557}