Move XferEffects class to GrCustomXfermode file

include/gpu/effects/GrCustomXfermode.h

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef GrCustomXfermode_DEFINED
+#define GrCustomXfermode_DEFINED
+
+#include "SkXfermode.h"
+
+class GrFragmentProcessor;
+class GrTexture;
+
+/**
+ * Custom Xfer modes are used for blending when the blend mode cannot be represented using blend
+ * coefficients. It is assumed that all blending is done within the processors' emit code. For each
+ * blend mode there should be a matching fragment processor (used when blending with a background
+ * texture) and xfer processor.
+ */
+namespace GrCustomXfermode {
+    bool IsSupportedMode(SkXfermode::Mode mode); 
+
+    GrFragmentProcessor* CreateFP(SkXfermode::Mode mode, GrTexture* background);
+};
+
+#endif