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V4L2VEA has been able to work in Zero-Copy mode when a given
VideoFrame's dimension is the same as a resolution configured in
Initialize(). This is sufficient to enable Zero-Copy capture in
ARC++ case.

To enable Zero-Copy capture in chrome case, it is necessary to
scale VideoFrame for Simulcast case. This CL enables V4L2VEA
to scale VideoFrame by using ImageProcessor interface.

See https://tinyurl.com/frame-dimension-in-VEA
for how V4L2VEA works.

Bug: chromium:1033799, chromium:982201
Test: Hangout Meet on scarlet (with crrev.com/c/1923767 and crrev.com/c/2007802)
Test: VEA test on kukui
Change-Id: I07d5abfca0f5dd7c4adcfc10191316c5ba3021de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2010466
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740587}