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Previously, VA configuration was tied to VaapiDevice, meaning
that the VAProfile was static per VaapiDevice. This also
forced users of decode_test to specify the profile and codec,
i.e. VP9Profile0, even though the decoder can deduce both.

Separate out ScopedVAConfig and introduce SharedVAContext so that
decoders can manage this configuration and the configuration can
manage its own lifetime, allowing decode_test to be used on most
streams that change profile or resolution mid-stream.

BUG=chromium:1062407,b:175909299
TEST=./decode_test -video=resolution_change_500frames-vp9.ivf -out-prefix=res_change/frame on nautilus

Change-Id: I32ad5dfab57851166ce31844f6609784fbcaf900
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2358989
Commit-Queue: Jao-ke Chin-Lee <jchinlee@chromium.org>
Reviewed-by: Andres Calderon Jaramillo <andrescj@chromium.org>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838985}