Expose scm commands and executive.call as needed for auto-rebaseline.

Also, change fetch_layout_test_results to fetch failing_results.json.
full_results.json now includes every test run and is more than we need/want
for auto-rebaseline, which is the only consumer of this.

See https://codereview.chromium.org/18882003/ for the auto-rebaseline patch
in progress.

R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/19755008

git-svn-id: svn://svn.chromium.org/blink/trunk@154678 bbb929c8-8fbe-4397-9dbb-9b2b20218538