Add AudioOutputStream::Flush

Flush() is propagated from AudioRendererImpl to AudioOutputStream
and is used to signal that buffered data can be cleared.  On Cast,
we plan to buffer data in the AudioOutputSteam and we need an signal
form the renderer to flush buffered data when seeking and changing tracks.

Bug: b/124402185
Change-Id: Icca917cc8afa7a77419e06c562767d617df36bdb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1604331
Commit-Queue: Alex Leung <alexleung@google.com>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662013}
80 files changed