Create fragment processor for performing input color blend with child processor

include/gpu/effects/GrXfermodeFragmentProcessor.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 GrXfermodeFragmentProcessor_DEFINED
 #define GrXfermodeFragmentProcessor_DEFINED
 
 #include "SkXfermode.h"
 
 class GrFragmentProcessor;
 
 namespace GrXfermodeFragmentProcessor {
+    /** The color input to the returned processor is treated as the src and the passed in processor
+        is the dst. */
+    const GrFragmentProcessor* CreateFromDstProcessor(const GrFragmentProcessor* dst,
+                                                      SkXfermode::Mode mode);
+
+    /** The color input to the returned processor is treated as the dst and the passed in processor
+        is the src. */
+    const GrFragmentProcessor* CreateFromSrcProcessor(const GrFragmentProcessor* src,
+                                                      SkXfermode::Mode mode);
+
     const GrFragmentProcessor* CreateFromTwoProcessors(const GrFragmentProcessor* src,
                                                        const GrFragmentProcessor* dst,
                                                        SkXfermode::Mode mode);
 };
 
 #endif