The start code for H.262 codec-specific data may be across a packet boundary.
Before this change the offset passed to CsdBuffer.onData may have been before
the start point of the data in the newData buffer.
After this change, start codes are added directly to the CSD buffer when it's
filling and any start code bytes added by onData (at the end of a packet) are
discarded.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=163943584
