[MappableSI] Convert VideoFrameStructTraitsTest to use MappableSI.

Remove an existing test
VideoFrameStructTraitsTest::GpuMemoryBufferSharedImageVideoFrame and add
a new test VideoFrameStructTraitsTest::MappableSharedImageVideoFrame
which tests MappableSI backed VideoFrames.

Bug: 416304898
Change-Id: I17a808a97383ced119b58db677f073512bdc8d9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6521081
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Auto-Submit: vikas soni <vikassoni@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Commit-Queue: vikas soni <vikassoni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1457310}