Change return type of FaviconTabHelper::GetFavicon() to gfx::Image.

ui/gfx/image/image.h

 // Copyright (c) 2012 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.
 
 // An Image wraps an image any flavor, be it platform-native GdkBitmap/NSImage,
 // or a SkBitmap. This also provides easy conversion to other image types
 // through operator overloading. It will cache the converted representations
 // internally to prevent double-conversion.
 //
 // The lifetime of both the initial representation and any converted ones are
@@ skipping 89 matching lines @@
   // the Image.
   const SkBitmap* ToSkBitmap() const;
   const ImageSkia* ToImageSkia() const;
 #if defined(TOOLKIT_GTK)
   GdkPixbuf* ToGdkPixbuf() const;
   CairoCachedSurface* const ToCairo() const;
 #elif defined(OS_MACOSX)
   NSImage* ToNSImage() const;
 #endif
 
+  // Same as ToSkBitmap(), but returns a null SkBitmap if this image is empty.
+  SkBitmap AsBitmap() const;
+
+  // Same as ToSkBitmap(), but returns a ImageSkia with a null SkBitmap if this
+  // image is empty.
+  ImageSkia AsImageSkia() const;
+
   // Performs a conversion, like above, but returns a copy of the result rather
   // than a weak pointer. The caller is responsible for deleting the result.
   // Note that the result is only a copy in terms of memory management; the
   // backing pixels are shared amongst all copies (a fact of each of the
   // converted representations, rather than a limitation imposed by Image) and
   // so the result should be considered immutable.
   ImageSkia* CopyImageSkia() const;
   SkBitmap* CopySkBitmap() const;
 #if defined(TOOLKIT_GTK)
   GdkPixbuf* CopyGdkPixbuf() const;
@@ skipping 36 matching lines @@
   // be cheaply copied.
   scoped_refptr<internal::ImageStorage> storage_;
 
   friend class ::ImageTest;
   friend class ::ImageMacTest;
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_IMAGE_IMAGE_H_