WebRTC fetch recipe: Always sync branch heads.

This will make gclient download all the remote branches of
the WebRTC Git repo when the 'fetch webrtc' recipe runs.
This will result in a new fetch line in the .git/config for
branch-heads, like this:
[remote "origin"]
  url = https://chromium.googlesource.com/external/webrtc.git
  fetch = +refs/heads/*:refs/remotes/origin/*
  fetch = +refs/branch-heads/*:refs/remotes/branch-heads/*

In order to get the same setup for an existing checkout, do:
gclient sync --with_branch_heads
git fetch origin

BUG=437776
TESTED=Ran 'fetch webrtc' and 'fetch webrtc_ios' and verified
the remote branches were available with 'git branch --remotes'

R=jochen@chromium.org

Review URL: https://codereview.chromium.org/1030573002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294528 0039d316-1c4b-4281-b951-d872f2087c98

recipes/webrtc.py

@@ -27,6 +27,7 @@ class WebRTC(recipe_util.Recipe):
         },
       ],
       'auto': True,  # Runs git auto-svn as a part of the fetch.
+      'with_branch_heads': True,
     }
 
     if props.get('target_os'):