media/gpu/VFMapper: Return the currently allowed pixel format for Map()

We create VideoFrameMapper with a pixel format. Only this pixel format is when
calling Map(). VFMapper client may want to know the currently allowed pixel
format for Map(). This CL adds the function to VideoFrameMapper interface that
returns the pixel format.

Bug: 952147
Test: video_decode_accelerator_tests
Change-Id: I8a48636a11b898fb39a6bbed6276cebda85e6161
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1609726
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661277}
6 files changed