MSE: Reduce spurious discontinuities caused by partial append window start overlaps

MSE: Reduce spurious discontinuities caused by partial append window start overlaps

If a frame overlaps appendWindowStart, partial append window filtering
is supported for the track, and the frame is appended to the track
buffer, this change updates the track buffer's last frame duration with
the frame's original duration instead of the possibly trimmed duration.
If the trimmed duration were used, then the discontinuity detection
logic could detect a discontinuity on the next frame for the track where
there would otherwise not be a discontinuity.

BUG=381114
R=acolwell@chromium.org
TEST=FrameProcessorTest.PartialAppendWindowFilterNoDiscontinuity

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=277976

[wolenetz, 2014-06-18 00:04:27]

acolwell@, PTAL @ PS1:

I found no regression specific to this change on a release build against the
yt-dash-mse conformance unit test page.

No MSE layout regression, either.

With this change, the repro site in http://crbug.com/381114
(http://sr-repro.s3.amazonaws.com/gravitymux/repro-gravitymuxed.html) no longer
requires a seek to ~2s to play from that time. It shows a single buffered range
and plays from 0s..~139s, then $('video').ended evaluates to true, so this
change helps that site even though it has frequent DTS > PTS.

Without the frame_processor.cc change, the new unit test fails only the
SequenceMode variant. Logs show both sequence and segments variants trigger
FrameProcessor's discontinuity logic, but apparently SourceBufferStream merges
the segments-mode actually-discontinuous ranges and passes the test.

[acolwell GONE FROM CHROMIUM, 2014-06-18 00:32:15]

lgtm

[wolenetz, 2014-06-18 01:08:48]

The CQ bit was checked by wolenetz@chromium.org

[commit-bot: I haz the power, 2014-06-18 01:11:26]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/wolenetz@chromium.org/344513002/1

[commit-bot: I haz the power, 2014-06-18 07:45:29]

Change committed as 277976