Add gpu backend for SkPixelXorXfermode

src/gpu/glsl/GrGLSLXferProcessor.h

Index: src/gpu/glsl/GrGLSLXferProcessor.h
diff --git a/src/gpu/glsl/GrGLSLXferProcessor.h b/src/gpu/glsl/GrGLSLXferProcessor.h
index ed8099b6f307dedf121ede0a4ec2c2aaf3566278..3806549b4a097418948bdc1be83d92dff4fa86e0 100644
--- a/src/gpu/glsl/GrGLSLXferProcessor.h
+++ b/src/gpu/glsl/GrGLSLXferProcessor.h
@@ -68,6 +68,14 @@ public:
      */
     void setData(const GrGLSLProgramDataManager& pdm, const GrXferProcessor& xp);
 
+protected:
+    static void DefaultCoverageModulation(GrGLSLXPFragmentBuilder* fragBuilder,
+                                          const char* srcCoverage,
+                                          const char* dstColor,
+                                          const char* outColor,
+                                          const char* outColorSecondary,
+                                          const GrXferProcessor& proc);
+    
 private:
     /**
      * Called by emitCode() when the XP will not be performing a dst read. This method is