Store discard request on the opList and remove GrDiscardOp (take 3)

Change-Id: Ided58e0110b0b4e611ab65f46c18a6ae2b85f1bc
Reviewed-on: https://skia-review.googlesource.com/39340
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
10 files changed