Add include guards to header files in content missing them

content/child/image_decoder.h

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#ifndef CONTENT_CHILD_IMAGE_DECODER_H_
+#define CONTENT_CHILD_IMAGE_DECODER_H_
+
 #include <stddef.h>
 
 #include <vector>
 
 #include "base/macros.h"
 #include "ui/gfx/geometry/size.h"
 
 class SkBitmap;
 
 namespace content {
@@ skipping 22 matching lines @@
       const unsigned char* data, size_t size);
 
  private:
   // Size will be empty to get the largest possible size.
   gfx::Size desired_icon_size_;
 
   DISALLOW_COPY_AND_ASSIGN(ImageDecoder);
 };
 
 }  // namespace content
+
+#endif  // CONTENT_CHILD_IMAGE_DECODER_H_