WebRTC: Add fetch recipes to depot_tools.

For Mac, we're depending on having both ios+mac for target_os.

R=agable@chromium.org

Review URL: https://codereview.chromium.org/753123003

git-svn-id: http://src.chromium.org/chrome/trunk/tools/depot_tools@293085 4ff67af0-8c30-449e-8e8b-ad334ec8d88c