Required by the image-diff plugin.
There does not seem to be any easy way to add bower dependencies inside
plugins without adding them to core.
Building image-diff never seemed to work for a normal user. Im not
entirely sure how google got it working.
Bug: Issue 9911
Change-Id: I08499c86e236a1c3861b78ba698dbd4a3bc811d8
cb76de7e17