Currently Chromium's WebGL implementation doesn't work with the multithreaded compositor. In order to make it do so, minimally it will be necessary to add another color buffer so that the submission of WebGL rendering results to the compositor can be a true buffer swap. It might be worth adding two more buffers so that the rendering thread can submit results without ever blocking on the compositor and vice versa, though that would be overkill for the initial implementation.
Iain has been thinking about how to reduce the backbuffer requirements for canvas layers recently. It would be pretty cool if we could have a 1 solution for both cases instead of 2. :)
*** This bug has been marked as a duplicate of bug 80459 ***