Avoid syncing unnecessary Chromium deps for WebRTC.

This should save several gigabytes of traffic and disk space.

On Linux this is about 2.6 GB:
346M	src/chrome/tools/test/reference_build
340M	src/native_client
170M	src/third_party/ffmpeg
1.5G	src/third_party/WebKit
196M	src/v8

BUG=2863
TESTED=Removed the directories locally, ran a sync and verified they didn't reappear (or fail because of platform-specific ones).
R=iannucci@chromium.org, niklas.enbom@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/22189004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6984 4adac7df-926f-26a2-2b94-8c16560cd09d