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This CL extracts the call to vaCreateContext() out of VaapiWrapper's
CreateSurfaces() and into a new method VaapiWrapper::CreateContext().

The former still calls the latter, but this change allows for separating
the allocation of the surfaces from the creation of the context. In
particular, this is needed for decoding directly on client Surfaces
(see the bug and/or the experimental CL crrev.com/c/986353. from
which this code is separated).

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: I37f28d2a0fe939264d3153f34bd25bbc6c77ba79
Reviewed-on: https://chromium-review.googlesource.com/995623Reviewed-by: Kristian H. Kristensen <hoegsberg@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548515}