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This CL adds silence detection logic for AudioCapturerMac so that it
doesn't send empty audio packets and waste the bandwidth.

Bug: 1161363
Change-Id: I891355b0ca31fb12eb40feb4f9b189645d36193c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618473Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841737}