Enable the Sky compositor again

This CL fixes two bugs:

1) We need to enter the Ganesh context in order to destroy the GrContext.

2) We had a race condition whereby we'd try to upload a frame to a surface that
   didn't yet exist. This CL fixes that race by adding a state to LayerHost to
   wait for the surface to be created before trying to upload to it.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/753643002