For simplicity, the canned presubmit check just calls git cl format
directly with a new option --dry-run which does not change the files on
disk.

Because some users may not have git or clang-format might somehow have
errors, this canned check is a warning.  Additionally, if git cl format
fails with an error then the presubmit check will silently report
success to avoid spamming misconfigured users during upload.

BUG=none

Review URL: https://codereview.chromium.org/141493002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@246066 0039d316-1c4b-4281-b951-d872f2087c98