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This introduces VideoFrameMapper for GpuMemoryBuffer-based VideoFrame. It is
required for tests to map the frame for verification and writing into a file.

Bug: 1011281
Test: image_processor_test --save_images on atlas
Change-Id: Ie3963e4c7185096d23a5a30507515a3f32144ee2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865051
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: David Staessens <dstaessens@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708473}