gpu/vp9_dec: Use VP9ReferenceFraameVector for ref pic management

VP9ReferenceFraameVector is the class implemented
for generalising the VP9 reference picture mangament with in
the encoder and decoder similar to the VP8 implementation.
Currenlty VP9ReferenceFraameVector is only utilized in the encoder
implementation. This patch enables the same in decoder too.

Bug:chromium:924804
TEST= video_decode_accelerator_unittest \
    --test_video_data=test-25fps.vp9:320:240:250:250:35:150:12 \
    -v=0 --disable_flush --single-process-tests --ozone-platform=gbm

Change-Id: I725c214c38a5aed1720bd9d91445ac4841f5014b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1540482
Commit-Queue: Sreerenj Balachandran <sreerenj.balachandran@intel.com>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#647129}

• media/gpu/BUILD.gn[diff]
• media/gpu/v4l2/v4l2_vp9_accelerator.cc[diff]
• media/gpu/v4l2/v4l2_vp9_accelerator.h[diff]
• media/gpu/vaapi/vaapi_vp9_accelerator.cc[diff]
• media/gpu/vaapi/vaapi_vp9_accelerator.h[diff]
• media/gpu/vp9_decoder.cc[diff]
• media/gpu/vp9_decoder.h[diff]
• media/gpu/windows/d3d11_vp9_accelerator.cc[diff]
• media/gpu/windows/d3d11_vp9_accelerator.h[diff]