Revert "Store discard request on the opList and remove GrDiscardOp"

This reverts commit b681a0f1b0acebe36130fd463d14016d48295b97.

Reason for revert: Seems to be messing up some MacMini & Nexus7 bots
Original change's description:
> 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>

TBR=egdaniel@google.com,bsalomon@google.com,robertphillips@google.com

Change-Id: I8a89fae7bb11791bd023d7444a074bb34d006fd0
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/32704
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
10 files changed