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 00acd34d9d52cd39c8b2a94d134fe059e16b453a..8f2932d83c1008e7e7f360abcdbcd4e470c48e4d 100644
--- a/src/gpu/gl/GrGLGpu.h
+++ b/src/gpu/gl/GrGLGpu.h
@@ -218,6 +218,11 @@ private:
                        const SkIRect& srcRect,
                        const SkIPoint& dstPoint) override;
 
+    void onGetMultisampleSpecs(GrRenderTarget*,
+                               const GrStencilSettings&,
+                               int* effectiveSampleCnt,
+                               SamplePattern*) override;
+
     // binds texture unit in GL
     void setTextureUnit(int unitIdx);