Will be used to provide proper ThreadTaskRunnerHandle context in unit
tests when multiple test task runners share the main thread.

TestMockTimeTaskRunner::ScopedContext as discussed @
https://groups.google.com/a/chromium.org/d/msg/chromium-dev/kSIe9p5ZYGM/koFefJmNCAAJ

This cleans up HttpServerPropertiesManagerTest to actually run code
in the scope of the task runners the impl expects it to (it would
previously pass because TestMockTimeTaskRunner::RunsOnCurrentThread()
is looser than it should be -- merely checks thread id, not runner context).
This is a prerequisite for https://codereview.chromium.org/2491613004/
as it otherwise breaks per its tasks posting to unexpected tasks runners
(i.e. the current ThreadTaskRunnerHandle() which was the underlying unmocked
MessageLoop in these tests).

Further addressed in those tests:
 - No longer need to use MessageLoop/RunLoop
   - Which removes need for completion Closure on the updates.
 - Exposes timer timings to fast-forward by the right timing instead of
   FastForwardUntilNoTasksRemain() -- test could use some more cleanup in that
   regard but that's a first pass of mandatory sites.

Also addressed in this CL:
 - ServerBackedStateKeysBrokerTest.Refresh, RecentTabHelperTest, AffiliatedMatchHelperTest, and SyncStoppedReporterTest
  - Now mocks main thread runner directly, removes need for test-only SetTaskRunner() methods :).
   (having a custom TaskRunner was causing things like observers to post to the wrong task runner per this CL's
    ThreadTaskRunnerHandle override in its deferred initialization).
 - AsyncDocumentSubresourceFilterTest, WallPaperColorCalculatorTest and PostAndReplyImplTest
  - Unfortunately those tests were fine but as documented in thread_task_runner_handle.cc:
    supporting SequencedTaskRunnerHandle overrides from main thread would be overkill for
    now... so they were tweaked to avoid doing that.
  - Also, upcoming base::test::ScopedTaskEnvironment makes all of this an implementation detail anyways:
    draft @ https://docs.google.com/document/d/1QabRo8c7D9LsYY3cEcaPQbOCLo8Tu-6VLykYXyl3Pkk/edit

BUG=587199

Review-Url: https://codereview.chromium.org/2657013002
Cr-Commit-Position: refs/heads/master@{#454127}