ash: Avoid implicit conversion from ImageSkia into SkBitmap.

ash/desktop_background/desktop_background_controller.cc

 // 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.
 
 #include "ash/desktop_background/desktop_background_controller.h"
 
 #include "ash/desktop_background/desktop_background_view.h"
 #include "ash/desktop_background/desktop_background_widget_controller.h"
 #include "ash/shell.h"
 #include "ash/shell_factory.h"
@@ skipping 106 matching lines @@
     return current_wallpaper_->wallpaper_image;
   return gfx::ImageSkia();
 }
 
 WallpaperLayout DesktopBackgroundController::GetWallpaperLayout() const {
   if (current_wallpaper_.get())
     return current_wallpaper_->wallpaper_layout;
   return CENTER_CROPPED;
 }
 
-SkBitmap DesktopBackgroundController::GetCurrentWallpaperImage() {
+gfx::ImageSkia DesktopBackgroundController::GetCurrentWallpaperImage() {
   if (desktop_background_mode_ != BACKGROUND_IMAGE)
-    return SkBitmap();
+    return gfx::ImageSkia();
   return GetWallpaper();
 }
 
 void DesktopBackgroundController::OnRootWindowAdded(
     aura::RootWindow* root_window) {
   // Handle resolution change for "built-in" images."
   if (BACKGROUND_IMAGE == desktop_background_mode_) {
     if (current_wallpaper_.get()) {
       gfx::Size root_window_size = root_window->GetHostSize();
       int wallpaper_width = current_wallpaper_->wallpaper_image.width();
@@ skipping 200 matching lines @@
        iter != root_windows.end(); ++iter) {
     gfx::Size root_window_size = (*iter)->GetHostSize();
     if (root_window_size.width() > kSmallWallpaperMaximalWidth ||
         root_window_size.height() > kSmallWallpaperMaximalHeight)
       resolution = LARGE;
   }
   return resolution;
 }
 
 }  // namespace ash