Enable blend optimizations with instanced draws

src/gpu/gl/GrGLGpu.h

Index: src/gpu/gl/GrGLGpu.h
diff --git a/src/gpu/gl/GrGLGpu.h b/src/gpu/gl/GrGLGpu.h
index 013613ad272c59b49ff4a361c5c1d5c7a2bcd96a..b7daa431d846360ce067de22a977d21931c5558b 100644
--- a/src/gpu/gl/GrGLGpu.h
+++ b/src/gpu/gl/GrGLGpu.h
@@ -28,6 +28,8 @@ class GrPipeline;
 class GrNonInstancedMesh;
 class GrSwizzle;
 
+namespace gr_instanced { class GLInstancedRendering; }
+
 #ifdef SK_DEBUG
 #define PROGRAM_CACHE_STATS
 #endif
@@ -54,6 +56,8 @@ public:
         return static_cast<GrGLPathRendering*>(pathRendering());
     }
 
+    gr_instanced::InstancedRendering* createInstancedRenderingIfSupported() override;
+
     // Used by GrGLProgram to configure OpenGL state.
     void bindTexture(int unitIdx, const GrTextureParams& params, bool allowSRGBInputs,
                      GrGLTexture* texture);
@@ -597,6 +601,7 @@ private:
 
     typedef GrGpu INHERITED;
     friend class GrGLPathRendering; // For accessing setTextureUnit.
+    friend class gr_instanced::GLInstancedRendering; // For accessing flushGLState.
 };
 
 #endif