vaapi: minor cleanups in VaVDA (OutputPicture, mostly)

This CL is a few cleanups to ease review of crrev.com/c/986353:

- PictureById() is moved to the end of the cc file, following the
 order of its declaration. Code untouched (except s/NULL/nullptr/).
 A call to PictureById  is substituted with base::ContainsKey()
 to better reflect intent.

- OutputPicture() loses its last parameter, that is then calculated
 on the spot (it's |available_picture_buffers_.front()|). This makes
 OutputCB unnecessary, so it's made a Closure, and a OnceClosure at
 that.

Test: eve crosvideo vp9 cycling resolutions.

Bug: 822346
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ie17f23f2fb20752463e9ebeacd1bf7a9b6150437
Reviewed-on: https://chromium-review.googlesource.com/1000297
Reviewed-by: Kristian H. Kristensen <hoegsberg@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549506}
2 files changed