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Previously we were casting away const at call sites because vaCreateBuffer
accepts a non-const buffer. However its documentation says that the data
will be copied out, and the implementation does so as well.
We can avoid this by mapping server-side memory via vaMapBuffer and copying
it ourselves, as VaapiWrapper::SubmitVAEncMiscParamBuffer does.

Bug: 844303
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: I4d66d0d6ee5e672d22a5dbe4073244515fa1b14d
Reviewed-on: https://chromium-review.googlesource.com/1080495
Commit-Queue: Sharif Elcott <selcott@chromium.org>
Reviewed-by: Pawel Osciak <posciak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564878}