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This CL extracts a few cleanups/preparations of crrev.com/c/1171566
(that is a big CL and might get reverted a few times).

- VaapiPicture::va_surface_id() allows for accessing the underlying
 VASurfaceID, if this is present (otherwise we return VA_INVALID_ID).

- Pictures is made a base::small_map (originally crrev.com/c/988512),
 PictureById() is removed in favour of using base::ContainsKey()
 on 3 callsites.

Bug: 822346, 717265
Test: v_d_a_unittest vp8/vp9/h264 passing on nautilus
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: I18a0335462b321d25550d295cb628b4ddd3e2269
Reviewed-on: https://chromium-review.googlesource.com/1180295Reviewed-by: Emircan Uysaler <emircan@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584256}