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Some vaapi wrapper cleanup:

- Adapted singletons to follow recommendations:
  * Added DISALLOW_COPY_AND_ASSIGN
  * Get method returns (const)& instead of *
  * Use base::NoDestructor to construct the singletons
- Made VASupportedProfiles member functions const

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: Ib9b11bd42d2c7c64ea5c72e649a9504bab8014ba
Reviewed-on: https://chromium-review.googlesource.com/1143063Reviewed-by: Miguel Casas <mcasas@chromium.org>
Commit-Queue: David Staessens <dstaessens@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580072}