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This is the replacement for GLHelper::I420Converter. It uses the new
GLScaler to efficiently scale and re-format RGBA textures into I420
image planes. In a soon-upcoming CL, GLRendererCopier will be switched
over to use this.

Includes unit and pixel testing.

Bug: 870036, 810131
Change-Id: I465eb933e7eadeb205676211432cced361a3739b
Reviewed-on: https://chromium-review.googlesource.com/c/1330830Reviewed-by: Ria Jiang <riajiang@chromium.org>
Commit-Queue: Yuri Wiitala <miu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608182}