Storage Monitor: Migrate Media Storage Util to TaskScheduler

This also required adding an optional ScopedTaskEnvironment to
RenderViewHostTestHarness. It's usually completely harmless, and
compatible with the existing TestBrowserThreadBundle.

However, in one test (AudioRendererHostTest), the MediaStreamManager
relies on TestBrowserThreadBundle behavior in the absence of
ScopedTaskEnvironment. We disable the ScopedTaskEnvironment in that
case.

Bug: 689520
Change-Id: Id170baac5136eff95972b86790ba9092a6db07be
Reviewed-on: https://chromium-review.googlesource.com/575383
Commit-Queue: Tommy Li <tommycli@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489718}
7 files changed