Store discard request on the opList and remove GrDiscardOp (take 2)

Change-Id: I2f1bd6f8547895cc8d66cfde42d7d890441d198e
Reviewed-on: https://skia-review.googlesource.com/33460
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
9 files changed