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Since FFmpeg decode of 0-byte audio and video buffers is now disallowed,
and FFmpegDemuxer skips enqueuing such packets into its DemuxerStream,
this change does similar for MSE:

Any 0-byte |data| audio or video buffers are dropped during initial
processing in FrameProcessor::ProcessFrames(). No 0-byte |data| text
buffers are dropped, because:
* We don't use FFmpeg to decode in-band MSE text, and
* In-band MSE text buffers' |side_data| may contain meaningful
  information. (This assumption isn't asserted by this change).

Note, this change takes the simpler route of dropping 0-byte AV buffers
before they're buffered by the coded frame processing algorithm. A more
complex alternative to skip them when reading from SourceBufferStream
was rejected in hopes this simpler change is sufficient.

Includes ability in ChunkDemuxerTest to vary |block_size_| instead of
always using |kBlockSize|. While the product change is closer to
FrameProcessor, FrameProcessorTests always encode the coded frame's
timestamp as the coded frame contents to enable precise verification of
processing results, so the test of this change is done via
ChunkDemuxerTest instead.

BUG=823375,663438
TEST=ChunkDemuxerTest.ZeroLengthFramesDropped

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ie6a802ba64a576f0219ca7c400fc7dec3a45fbf6
Reviewed-on: https://chromium-review.googlesource.com/974427
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546159}