Invalidate all wallpaper cache if display configuration changed

ash/desktop_background/user_wallpaper_delegate.h

 // Copyright 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.
 
 #ifndef ASH_DESKTOP_BACKGROUND_USER_WALLPAPER_DELEGATE_H_
 #define ASH_DESKTOP_BACKGROUND_USER_WALLPAPER_DELEGATE_H_
 
 #include "ash/ash_export.h"
 #include "ui/views/corewm/window_animations.h"
 
@@ skipping 13 matching lines @@
 
   // Sets wallpaper animation duration in ms. Pass 0 to use the default.
   virtual void SetAnimationDurationOverride(int animation_duration_in_ms) = 0;
 
   // Should the slower initial animation be shown (as opposed to the faster
   // animation that's used e.g. when switching from one user's wallpaper to
   // another's on the login screen)?
   virtual bool ShouldShowInitialAnimation() = 0;
 
   // Updates current wallpaper. It may switch the size of wallpaper based on the
-  // current display's resolution.
-  virtual void UpdateWallpaper() = 0;
+  // current display's resolution. If |clear_cache| is true, all wallpaper
+  // cache should be cleared. This is required when the display's native
+  // resolution changes to a larger resolution (e.g. when hooked up a large
+  // external display) and we need to load a larger resolution wallpaper for the
+  // large display. All the previous small resolution wallpaper cache should be
+  // cleared.
+  virtual void UpdateWallpaper(bool clear_cache) = 0;
 
   // Initialize wallpaper.
   virtual void InitializeWallpaper() = 0;
 
   // Opens the set wallpaper page in the browser.
   virtual void OpenSetWallpaperPage() = 0;
 
   // Returns true if user can open set wallpaper page. Only guest user returns
   // false currently.
   virtual bool CanOpenSetWallpaperPage() = 0;
 
   // Notifies delegate that wallpaper animation has finished.
   virtual void OnWallpaperAnimationFinished() = 0;
 
   // Notifies delegate that wallpaper boot animation has finished.
   virtual void OnWallpaperBootAnimationFinished() = 0;
 };
 
 }  // namespace ash
 
 #endif  // ASH_DESKTOP_BACKGROUND_USER_WALLPAPER_DELEGATE_H_