[WebM] Seen a lot of small gaps when trying to package an existing WebM input

[kqyang]

Test file: https://commons.wikimedia.org/wiki/File:Sintel_webm_extract.webm

[0120/102238:WARNING:mpd_builder.cc(1259)] Found a gap of size 170000 > kRoundingErrorGrace (5). The new segment starts at 10667000 but the previous segment ends at 10497000.
[0120/102238:WARNING:mpd_builder.cc(1259)] Found a gap of size 168000 > kRoundingErrorGrace (5). The new segment starts at 21167000 but the previous segment ends at 20999000.
[0120/102238:WARNING:mpd_builder.cc(1259)] Found a gap of size 165000 > kRoundingErrorGrace (5). The new segment starts at 31500000 but the previous segment ends at 31335000.
[0120/102238:WARNING:mpd_builder.cc(1259)] Found a gap of size 164000 > kRoundingErrorGrace (5). The new segment starts at 41750000 but the previous segment ends at 41586000.
...

Investigated into the problem, and found out that: we set the duration for every sample to track level default duration; however, the actual duration or more precisely, the time stamp difference between two consecutive samples are not exactly the same as track level default duration. The difference is small for each individual samples, e.g. 42ms vs 41ms. However, the difference is accumulated and grows to a large value. That is what we are seeing in the warning.

To fix the problem, we should not use track level default duration. Instead, we should compute the duration from next sample instead.

[kqyang]

Fixed.