clang-format: Allow .proto files to be formatted (supported by an upcoming binary/script; this change is safe if you are running an older binary); remove warnings.

BUG=344916

Review URL: https://codereview.chromium.org/177213013

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@253295 0039d316-1c4b-4281-b951-d872f2087c98