H264Parser::decode_Header: pic_order_present_flag is not required, so don't
insist on it.

(cherry picked from commit faf7487bd3db2f01d273e1bd7a44cf64f7762181)
DTVRecorder::FindH264Keyframes: Only account for Adaptation Field Control
Offset if payloadStart is set.

(cherry picked from commit 6c6c0cbd4d95957c107e0e7e37b6b4cf70b04de5)