Remove checkout functionality from GOM

Some folks still probably run GOM locally, but
it's not worth the burden on the code to keep this around.
If we want this we should re-build it to be web only.

R=abarth@chromium.org, abarth, dpranke
BUG=389681

Review URL: https://codereview.chromium.org/355193005

git-svn-id: svn://svn.chromium.org/blink/trunk@177140 bbb929c8-8fbe-4397-9dbb-9b2b20218538