WebRTC fetch recipe: Always sync branch heads.
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: bd64dd6fa6f3f0ed0c0666d1018379882b742947@294528 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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