Make fetch chromium operate in git-only mode.
Make fetch chromium operate in git-only mode.

DO NOT LAND until Flag Day.

R=stip@chromium.org
BUG=406165

Review URL: https://codereview.chromium.org/494993006

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@291565 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools