Initial wireup of VideoBitrateAllocation in VaapiVideoEncodeAccelerator.

This is a first step just to get the allocation signal down into the
VP8 encoder. Temporal layer handling will follow upcoming CLs.

Bug: chromium:794608
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;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Ie0a120a7b906dc5218834e355d3acf8c4f24ac36
Reviewed-on: https://chromium-review.googlesource.com/1065898
Commit-Queue: Erik Språng <sprang@chromium.org>
Reviewed-by: Pawel Osciak <posciak@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561853}
11 files changed