SDK / exoplayer

When the ctts contained an entry that had a 0-valued entry count, the
parser would miss every other entry, failing the final assertion.
The standard does not seem to prevent the value 0 in the sample_count
field, so we need to allow it.

Issue: #1326
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=117241945