Store discard request on the opList and remove GrDiscardOp

Change-Id: Ic1f76bb91c16b23df1fe71c07a4d5ad5abf1dc26
Reviewed-on: https://skia-review.googlesource.com/32640
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
10 files changed