WebRTC: Update source-of-truth for 'git-cl'

Bug: 751635
Change-Id: I503dc5d108c314ef617af2e46e937bc63cd087b9
Reviewed-on: https://chromium-review.googlesource.com/660703Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

git_cl.py

@@ -966,6 +966,8 @@ def _is_git_numberer_enabled(remote_url, remote_ref):
       'external/webrtc',
       'v8/v8',
       'infra/experimental',
+      # For webrtc.googlesource.com/src.
+      'src',
   ]
 
   assert remote_ref and remote_ref.startswith('refs/'), remote_ref