Add Gold git main method Adds _GetGitOriginMainHeadSha1 to the Skia Gold code, which is equivalent to the existing _GetGitOriginMasterHeadSha1 except that it checks against origin/main. The former will be used and the latter removed once Chromium is able to safely update which version it calls. Bug: chromium:1276531 Change-Id: I23b2da0aac91a182fde934b7c7396e41fe6bf07c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3314883 Commit-Queue: Brian Sheedy <bsheedy@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>