Support multiple corpora for gtest pixel tests

Adds support for specifying a non-default corpus when initializing a
SkiaGoldPixelDiff or BrowserSkiaGoldPixelDiff instance. This allows
users to separate their images, either to reduce clutter when looking
through results or to prevent internal results from being mirrored to
the public Gold instance.

This particular implementation is meant to be a temporary solution so
that users can be unblocked. A more robust implementation that will
catch public corpora being used in non-public settings, etc. will be
implemented later.

Bug: 1108465
Change-Id: I8f868bfb32cdf72cf9feccf64c7291f758c3ae5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2314976
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Sven Zheng <svenzheng@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791277}