Add XferProcessor factory in GrPaint and GrDrawState.

src/gpu/effects/GrDefaultXferProcessor.h

+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef GrDefaultXferProcessor_DEFINED
+#define GrDefaultXferProcessor_DEFINED
+
+#include "GrPorterDuffXferProcessor.h"
+
+class GrDefaultXPFactory : public GrPorterDuffXPFactory {
+public:
+    static const GrXPFactory* Create() {
+        GR_CREATE_STATIC_XP_FACTORY(gDefaultXPFactory, GrDefaultXPFactory, ());
+        return SkRef(gDefaultXPFactory);
+    }
+
+private:
+    GrDefaultXPFactory() : INHERITED(SkXfermode::kOne_Coeff, SkXfermode::kZero_Coeff) {}
+
+    typedef GrPorterDuffXPFactory INHERITED;
+};
+
+#endif
+