Reland "Stack-allocate pipelines for GrMeshDrawOp"

This is a reland of dfe5000a5ff2cabb50ddc139882dc1f3005fa429

Original change's description:
> Stack-allocate pipelines for GrMeshDrawOp
>
> Stack-allocates the pipelines in onExecute. This saves us from having
> to store the pipelines on the heap, as well as delaying the need to
> detach processors until onExecute. The delay is an improvement because
> it allows us to keep visiting proxies after onPrepare. (Previously,
> they were moved out of GrProcessorSet and into a pipeline during
> onPrepare, so visiting proxies was impossible after that point.)
>
> Bug: skia:8731
> Change-Id: Idc05063fb0dfbfed42b434e429fa5a497097bdae
> Reviewed-on: https://skia-review.googlesource.com/c/193368
> Commit-Queue: Chris Dalton <csmartdalton@google.com>
> Reviewed-by: Robert Phillips <robertphillips@google.com>

TBR=robertphillips@google.com

Bug: skia:8731
Change-Id: I32def1a35bb0593470fa672691a9e697dc6d9680
Reviewed-on: https://skia-review.googlesource.com/c/195261
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
diff --git a/src/gpu/ops/GrRegionOp.cpp b/src/gpu/ops/GrRegionOp.cpp
index c050142..a90bfb9 100644
--- a/src/gpu/ops/GrRegionOp.cpp
+++ b/src/gpu/ops/GrRegionOp.cpp
@@ -134,8 +134,11 @@
                 iter.next();
             }
         }
-        auto pipe = fHelper.makePipeline(target);
-        helper.recordDraw(target, std::move(gp), pipe.fPipeline, pipe.fFixedDynamicState);
+        helper.recordDraw(target, std::move(gp));
+    }
+
+    void onExecute(GrOpFlushState* flushState, const SkRect& chainBounds) override {
+        fHelper.executeDrawsAndUploads(this, flushState, chainBounds);
     }
 
     CombineResult onCombineIfPossible(GrOp* t, const GrCaps& caps) override {