Rename presubmit.py to presubmit_support.py to reduce confusion and be able to add a PRESUBMIT.py check into depot_tools.
Review URL: http://codereview.chromium.org/113337

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@15954 0039d316-1c4b-4281-b951-d872f2087c98