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Adds the ability to pass in --code-review-system to specify the code
review system being used for a Skia Gold-backed gtest pixel test. This
defaults to "gerrit" if not passed in, which is what it was hard-coded
to before, but can now be set to "gerrit-internal" for internal code
reviews.

Bug: skia:10532
Change-Id: I28848158efbf83f223f5d56acdf5bd62271ce95b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2476669Reviewed-by: Sven Zheng <svenzheng@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818061}