Flush GrDirectContext directly

In http://review.skia.org/716476, Skia removed SkCanvas::flush()
so this replaces that call with a direct call to flushAndSubmit()
on the (known-to-be-non-null) GrDirectContext (the previous
behavior).

Change-Id: I6df29c328b2fcf5032d27bf65d87e50c2f04c3e0