GitLab

Merges various parts of the GPU Gold documentation from
gpu_pixel_testing_with_gold.md into the other parts of the GPU
documentation. Also removes references to the legacy pixel tests and
their triage process since the code for them has been removed.

Bug: 985007
Change-Id: I19d5480de77f1066046ef8fe0f076264cff01ee7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1721523
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Auto-Submit: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681868}