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Fixes a "choppy audio" issue on platforms with certain audio timing
parameters/scenarios. Investigation on crbug.com/934770 led to the
realization that three related problems needed to be addressed:

1. Fix the "gap" detection in audio::SnooperNode to check in BOTH
directions, not just one direction; and on both the input and output
flows. This accounts for the case where a device-switch in
audio::OutputController results in a sudden drastic shift in the delay
timestamps.

2. Add automatically-increasing capture delay logic to
audio::LoopbackStream to ensure the reads from all nodes' delay buffer
will never result in underrun.

3. Increased the accuracy of media::FakeAudioWorker/OutputStream by: a)
addressing a subtle source of error in task scheduling delay math due to
integer truncation; b) exposing timestamps to worker callbacks that
allow FakeAudioOutputStream to provide a |delay| and |delay_timestamp|
that behaves just like a real AudioOutputStream. This "lessens the blow"
of a device-switch to SnooperNode.

Bug: 934770
Change-Id: Ia75f18b2be3ad905f27d6ad882df9632764bb81c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1504981
Commit-Queue: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Max Morin <maxmorin@chromium.org>
Reviewed-by: Oskar Sundbom <ossu@chromium.org>
Reviewed-by: Weiyong Yao <braveyao@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638427}