Remove ImmediateFlush mode

Change-Id: I7db113e66d81516b2beb5eefeddf488d9bfed2e5
Reviewed-on: https://skia-review.googlesource.com/18488
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
8 files changed