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- Both AudioEncoders and WebmMuxer were using ThreadCheckers
  to assert operations being called on a valid thread. However,
  there's no need to enforce all operations happening on the
  exact same thread. What we care about is the sequence of
  operations, therefore a SequenceChecker is good enough.
- Add mechanisms to flush any buffered frames in the encoders
  or muxer to avoid dropping frames, and also enable manual
  flushing rather than having to destroy the object to have it
  flush its buffers.

BUG=1126586
TEST=Modified a test in media_unittests --gtest_filter=*AudioEncodersTest*

Change-Id: Ife44c440c48408553dacdd92fd8800cdc5c7a233
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490884Reviewed-by: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820432}