Enable blend optimizations with instanced draws

src/gpu/GrGpu.h

Index: src/gpu/GrGpu.h
diff --git a/src/gpu/GrGpu.h b/src/gpu/GrGpu.h
index a733f45e3c12ef79239b64e82d00060e4cac196a..032edc3fc97f75e1ebe1eedb26267e43ff318dff 100644
--- a/src/gpu/GrGpu.h
+++ b/src/gpu/GrGpu.h
@@ -38,6 +38,8 @@ class GrStencilSettings;
 class GrSurface;
 class GrTexture;
 
+namespace gr_instanced { class InstancedRendering; }
+
 class GrGpu : public SkRefCnt {
 public:
     /**
@@ -147,6 +149,13 @@ public:
                            const void* data = nullptr);
 
     /**
+     * Creates an instanced rendering object if it is supported on this platform.
+     */
+    virtual gr_instanced::InstancedRendering* createInstancedRenderingIfSupported() {
+        return nullptr;
+    }
+
+    /**
      * Resolves MSAA.
      */
     void resolveRenderTarget(GrRenderTarget* target);
@@ -579,6 +588,7 @@ private:
     GrContext*                                                          fContext;
 
     friend class GrPathRendering;
+    friend class gr_instanced::InstancedRendering;
     typedef SkRefCnt INHERITED;
 };