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

This reverts commit 9b0b32fda4871776eb9afdf9553e523e5c28aa63.

Reason for revert: Android_Vulkan Gold images are broken

Original change's description:
> 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>

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

Change-Id: I1470d88c2407864da8b6ebdc119c8c2466a2a6c6
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/39000
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index 57564e9..51ac125 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -247,6 +247,7 @@
   "$_src/gpu/ops/GrDashOp.h",
   "$_src/gpu/ops/GrDefaultPathRenderer.cpp",
   "$_src/gpu/ops/GrDefaultPathRenderer.h",
+  "$_src/gpu/ops/GrDiscardOp.h",
   "$_src/gpu/ops/GrDebugMarkerOp.h",
   "$_src/gpu/ops/GrDrawAtlasOp.cpp",
   "$_src/gpu/ops/GrDrawAtlasOp.h",