Change GetAllDisplays() in all screens to return a const vector&.

ui/display/screen.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.
 
 #ifndef UI_DISPLAY_SCREEN_H_
 #define UI_DISPLAY_SCREEN_H_
 
 #include <vector>
 
 #include "base/macros.h"
@@ skipping 35 matching lines @@
 
   // Returns the window at the given screen coordinate |point|.
   virtual gfx::NativeWindow GetWindowAtScreenPoint(const gfx::Point& point) = 0;
 
   // Returns the number of displays.
   // Mirrored displays are excluded; this method is intended to return the
   // number of distinct, usable displays.
   virtual int GetNumDisplays() const = 0;
 
   // Returns the list of displays that are currently available.
-  virtual std::vector<display::Display> GetAllDisplays() const = 0;
+  virtual const std::vector<display::Display>& GetAllDisplays() const = 0;
 
   // Returns the display nearest the specified window.
   // If the window is NULL or the window is not rooted to a display this will
   // return the primary display.
   virtual display::Display GetDisplayNearestWindow(
       gfx::NativeView view) const = 0;
 
   // Returns the display nearest the specified point. |point| should be in DIPs.
   virtual display::Display GetDisplayNearestPoint(
       const gfx::Point& point) const = 0;
@@ skipping 23 matching lines @@
 
  private:
   DISALLOW_COPY_AND_ASSIGN(Screen);
 };
 
 Screen* CreateNativeScreen();
 
 }  // namespace display
 
 #endif  // UI_DISPLAY_SCREEN_H_