Make GrGLShaderBuilder check whether GrEffect advertised that it would require the dst color or frag

src/core/SkXfermode.cpp

Index: src/core/SkXfermode.cpp
diff --git a/src/core/SkXfermode.cpp b/src/core/SkXfermode.cpp
index eb03ef958568919ff4e19f58170d9c37063389a5..7896061486df1e1bdc234fc16e7ea5549a91ebb1 100644
--- a/src/core/SkXfermode.cpp
+++ b/src/core/SkXfermode.cpp
@@ -1358,7 +1358,7 @@ public:
     GR_DECLARE_EFFECT_TEST;
 
 private:
-    XferEffect(SkXfermode::Mode mode) : fMode(mode) { this->setWillReadDst(); }
+    XferEffect(SkXfermode::Mode mode) : fMode(mode) { this->setWillReadDstColor(); }
     virtual bool onIsEqual(const GrEffect& other) const SK_OVERRIDE { return true; }
 
     SkXfermode::Mode fMode;