Implement support for KHR_blend_equation_advanced

src/gpu/GrXferProcessor.cpp

Index: src/gpu/GrXferProcessor.cpp
diff --git a/src/gpu/GrXferProcessor.cpp b/src/gpu/GrXferProcessor.cpp
index 42ac1531c13ae76291d1873c4c634fd55392aea9..243b9feb3f32edd6cee2386aaf10d91fb135f89b 100644
--- a/src/gpu/GrXferProcessor.cpp
+++ b/src/gpu/GrXferProcessor.cpp
@@ -42,7 +42,7 @@ bool GrXferProcessor::willNeedXferBarrier(const GrRenderTarget* rt,
         *outBarrierType = kTexture_GrXferBarrierType;
         return true;
     }
-    return false;
+    return this->onWillNeedXferBarrier(rt, caps, outBarrierType);
 }
 
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