This CL also fixes the behavior of VTVDA with regards to MMCO5, which
should be handled like an IDR for reordering purposes. This follows
(perhaps not obviously) from the spec definition of "picture order
count":

  'A variable that is associated with each coded field and each field of
  a coded frame and has a value that is non-decreasing with increasing
  field position in output order relative to the first output field of
  the previous IDR picture in decoding order or relative to the first
  output field of the previous picture, in decoding order, that contains
  a memory management control operation that marks all reference
  pictures as "unused for reference".'

This definition conveniently guarantees that when pic_order_cnt_type is
2, then decode order == presentation order (as one would hope).

BUG=webrtc:7010
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2661423002
Cr-Commit-Position: refs/heads/master@{#456889}