Bug 855178. Use WaitSync instead of ClientWaitSync in SharedSurfaceGL. r=jgilbert

This is better than waiting/spinning. WaitSync will make the GPU do the
waiting for us and we can continue on.

Banana bench before:
gameplay: 98.554 total seconds
gameplay: 70.097 JS seconds

after:
gameplay: 77.188 total seconds
gameplay: 70.33 JS seconds

--HG--
extra : rebase_source : 6eca32668d828a9f312500cfb900c53b107357a1