Implement support for KHR_blend_equation_advanced

src/gpu/GrGpu.h

Index: src/gpu/GrGpu.h
diff --git a/src/gpu/GrGpu.h b/src/gpu/GrGpu.h
index 3df651f1e631e0d666a6eeb0887b5849b0165cc0..dd43d91f7945978a7738641018fe19eb054fae4d 100644
--- a/src/gpu/GrGpu.h
+++ b/src/gpu/GrGpu.h
@@ -304,6 +304,9 @@ public:
                              const SkIRect& srcRect,
                              const SkIPoint& dstPoint) = 0;
 
+    // Called before certain draws in order to guarantee coherent results from dst reads.
+    virtual void setXferBarrier(GrXferBarrierType) = 0;
+
     struct DrawArgs {
         typedef GrDrawTarget::DrawInfo DrawInfo;
         DrawArgs(const GrPrimitiveProcessor* primProc,