PresubmitPromptOrNotify prints the message and prompts the user only
if the presubmit is on upload, not commit.

This is useful to provide upload-time sanity checks without blocking
the CL from being landed via the commit-queue.


Review URL: https://chromiumcodereview.appspot.com/12676031

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@190711 0039d316-1c4b-4281-b951-d872f2087c98