After days of diagnosing the issue, it seems this was all caused by two things:

1. The I420 converter should not be doing extra calculations to determine the source texture rect (flipping of Y coordinates is done upstream).

2. GL_SCISSOR_TEST was enabled (causing the lack of data, or green bars, at the top of the result). By luck, the draw operations were never being cropped before; but with texture size rounding, the scissor rect impacted the drawing ops for scaling.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f5e97c7c18f90cefd0948e547e2e4253e411f5b5

commit f5e97c7c18f90cefd0948e547e2e4253e411f5b5
Author: Yuri Wiitala <miu@chromium.org>
Date: Thu Jan 11 22:54:50 2018

GLRendererCopier: Fix for erroneous green bars in CopyOutputResult.

A recent change (commit 07fe0b6e0939a8aa223d8fb526a8040b39441858)
modified all RenderPass texture size allocations to be rounded-up in 64
pixel increments. This revealed problems in the GLRendererCopier impl,
which have been fixed by explicitly accounting for both flipped and non-
flipped source textures, and also ensuring that the GL scissor is
disabled during scaling draw operations.

Bug:  801007 
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel
Change-Id: Ia8309d9d882da8f0c8c7acdd7ddfe7c4ea311f49
Reviewed-on: https://chromium-review.googlesource.com/861154
Commit-Queue: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Xiangjun Zhang <xjz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528793}
[modify] https://crrev.com/f5e97c7c18f90cefd0948e547e2e4253e411f5b5/components/viz/common/gl_helper_scaling.cc
[modify] https://crrev.com/f5e97c7c18f90cefd0948e547e2e4253e411f5b5/components/viz/service/display/gl_renderer.cc
[modify] https://crrev.com/f5e97c7c18f90cefd0948e547e2e4253e411f5b5/components/viz/service/display/gl_renderer_copier.cc
[modify] https://crrev.com/f5e97c7c18f90cefd0948e547e2e4253e411f5b5/components/viz/service/display/gl_renderer_copier.h
[modify] https://crrev.com/f5e97c7c18f90cefd0948e547e2e4253e411f5b5/components/viz/service/display/gl_renderer_copier_pixeltest.cc
[modify] https://crrev.com/f5e97c7c18f90cefd0948e547e2e4253e411f5b5/components/viz/service/display/gl_renderer_copier_unittest.cc
[modify] https://crrev.com/f5e97c7c18f90cefd0948e547e2e4253e411f5b5/components/viz/service/display/renderer_pixeltest.cc