Fix compositor integration in ui/compositor

Our version of ui/compositor/ did not have https://crrev.com/301512,
which recent changes relied on. MojoContextProvider also did not
zero-initialize its MojoGLES2Context parameter so if a
MojoContextProvider instance was created and then destroyed before being
bound it would try to destroy its non-NULL but invalid MojoGLES2Context
parameter leading to a crash.

R=sky@chromium.org

Review URL: https://codereview.chromium.org/808003002