Add option to draw wireframe batch bounds

src/gpu/batches/GrDiscardBatch.h

Index: src/gpu/batches/GrDiscardBatch.h
diff --git a/src/gpu/batches/GrDiscardBatch.h b/src/gpu/batches/GrDiscardBatch.h
index aa443cf60341041c859862e4a1c816324710debe..65b6c4bf7d3ab850f0cb5cf0ce46a4088bfeb369 100644
--- a/src/gpu/batches/GrDiscardBatch.h
+++ b/src/gpu/batches/GrDiscardBatch.h
@@ -26,6 +26,7 @@ public:
     const char* name() const override { return "Discard"; }
 
     uint32_t renderTargetUniqueID() const override { return fRenderTarget.get()->getUniqueID(); }
+    GrRenderTarget* renderTarget() const override { return fRenderTarget.get(); }
 
     SkString dumpInfo() const override {
         SkString string;