Don't make dst copies when color stage requires dst but color writes are disabled.

src/gpu/GrDrawState.h

Index: src/gpu/GrDrawState.h
diff --git a/src/gpu/GrDrawState.h b/src/gpu/GrDrawState.h
index 230b2faa47b5d27954e896854472bd6824696021..80ad28eac5d794edafa6915fb425a2ed5bee4ca4 100644
--- a/src/gpu/GrDrawState.h
+++ b/src/gpu/GrDrawState.h
@@ -451,14 +451,7 @@ public:
     /**
      * Checks whether any of the effects will read the dst pixel color.
      */
-    bool willEffectReadDstColor() const {
-        for (int s = 0; s < kNumStages; ++s) {
-            if (this->isStageEnabled(s) && (*this->getStage(s).getEffect())->willReadDstColor()) {
-                return true;
-            }
-        }
-        return false;
-    }
+    bool willEffectReadDstColor() const;
 
     /// @}