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The code would previously copy SkImage data out and use libyuv to do RGB
->YUV conversion, which causes some unnecessary memory copies. This CL
use Skia's YUV420 readback API instead of doing an RGB readback and a
color space conversion afterwards.

This is verified via a demo which do 720P/30fps UserMedia->ImageBitmap->
VideoFrame->encode->decode->render. and from chrome::tracing, the
average cpu time of VideoFrame::Create is reduced from 6.462ms to2.24ms.
and the package power consumption is reduced from 8.03W to 6.02W, so
about 25%. The IA memory bandwidth is reduced about 29%.

Bug: 1102290
Test: Manually test on atlas
Change-Id: I709bef072102e9fe23e16d69a370641b969e47e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2494900Reviewed-by: Eugene Zemtsov <eugene@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Zhaoliang Ma <zhaoliang.ma@intel.com>
Cr-Commit-Position: refs/heads/master@{#825244}