Add hooks for GL_EXT_debug_marker in gpu

include/gpu/gl/GrGLExtensions.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLExtensions_DEFINED
 #define GrGLExtensions_DEFINED
 
@@ skipping 36 matching lines @@
     /**
      * Queries whether an extension is present. This will fail if init() has not been called.
      */
     bool has(const char[]) const;
 
     /**
      * Removes an extension if present. Returns true if the extension was present before the call.
      */
     bool remove(const char[]);
 
+    /**
+     * Adds an extension to list
+     */
+    void add(const char[]);
+
     void reset() { fStrings->reset(); }
 
     void print(const char* sep = "\n") const;
 
 private:
     bool                                fInitialized;
     SkAutoTDelete<SkTArray<SkString> >  fStrings;
 };
 
 #endif