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This is part 3/4 CLs to move the <audio>/<video> elements off of
a synchronous API that can lead to renderer hangs and premature
audio renderer errors.

This moves the AudioRendererSinkCache from AudioRendererMixerManger
to a base::NoDestructor instance in AudioDeviceFactory and stops
ARMM from using the cache at all. The users remaining are WebAudio
and WebRTC, we can reevaluate metrics on keeping it after.

Due to base::(SingleThread|Sequenced)TaskRunner::Get() being
deprecated for content/ I've had to switch to use base::PostTask to
generate a cleanup task runner. Unfortunately this doesn't guarantee
that it runs on the render thread, but since we're now using a static
instance, we can just always use base::Unretained and drop the WeakPtr
factory that was handling the cancellation of these tasks.

There's no point in the cache for ARMM since we're going to require
a sink to get a mixer. That sink will always be used to get output
device info first too and then reused for the mixer.

BUG=905506
TEST=updated tests, compiles.
R=olka

Change-Id: I03408753f974e4c6fb9c89270508e26689162002
Reviewed-on: https://chromium-review.googlesource.com/c/1347730
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612460}