Add "sample locations" feature to GrProcessor

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 56ffcb340433240d30f9f85e1f1a60be04c9ace7..1ad519a5cee27902014fab469a165a0935b1c45d 100644
--- a/src/gpu/gl/GrGLGpu.h
+++ b/src/gpu/gl/GrGLGpu.h
@@ -223,6 +223,11 @@ private:
                        const SkIRect& srcRect,
                        const SkIPoint& dstPoint) override;
 
+    void onGetMultisampleSpecs(GrRenderTarget*,
+                               const GrStencilSettings&,
+                               int* effectiveSampleCnt,
+                               SkAutoTDeleteArray<SkPoint>* sampleLocations) override;
+
     // binds texture unit in GL
     void setTextureUnit(int unitIdx);