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This CL removes the use of the ConditionVariables inside VaVDA and
uses instead PostTask()ing. This removes blocking the worker
thread, which allows for moving other tasks there, e.g. the
Surface Blit (as done in the follow up WIP CL crrev.com/c/947341)

For all that goodness, this CL:

- Refactors the waiting logic that on ToT is spread between
 GetCurrInputBuffer_Locked() and WaitForSurfaces_Locked() and
 simplifies it in DecodeTask() itself.

- Moves the |curr_input_buffer_| filling logic in
 GetCurrInputBuffer_Locked() into DecodeTask() proper.

- Reduces the scoped of the |lock_| in DecodeTask() (note that
 |curr_input_buffer_| and |decode_| are only used on the decoder
 thread -- added a .h comment).

Minor stuff: This CL also moves together the declaration of the
related member vars |input_buffers_|, |curr_input_buffer_| and
|available_va_surfaces_|

TEST=simplechrome+crosvideo (including seeks) on soraka, and
v_d_a_unittest with h264, vp8, vp9 (with dcheck_always_on=true).

Bug: 717265
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
Change-Id: Ib167115dd2a4ebaeaea3ba3b5a205779d659f479
Reviewed-on: https://chromium-review.googlesource.com/949283
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542955}