This fixes Valgrind errors while running TestWebGLElementArrayCache.
This also fixes the way we do content/canvas/test in the Makefiles, which had been broken by my attempt to enable compiled-code tests there (bug 732660).
Since I still don't know how to do compiled-code tests there, I've disabled them. At least we have the rest of content/canvas/test back.
Let's hope that no regression happened.