Make gfx::Rect class operations consistently mutate the class they are called on.

ash/desktop_background/desktop_background_view.cc

Index: ash/desktop_background/desktop_background_view.cc
diff --git a/ash/desktop_background/desktop_background_view.cc b/ash/desktop_background/desktop_background_view.cc
index 311532e0c8dc3aa0d6f0be1ba5d9757d8a8a0c1b..b88d17b3339387fe44568520df7edf3e1bb49bcf 100644
--- a/ash/desktop_background/desktop_background_view.cc
+++ b/ash/desktop_background/desktop_background_view.cc
@@ -127,7 +127,8 @@ void DesktopBackgroundView::OnPaint(gfx::Canvas* canvas) {
           RoundPositive(static_cast<double>(height()) / horizontal_ratio));
     }
 
-    gfx::Rect wallpaper_cropped_rect = wallpaper_rect.Center(cropped_size);
+    gfx::Rect wallpaper_cropped_rect = wallpaper_rect;
+    wallpaper_cropped_rect.ClampToCenteredSize(cropped_size);
     canvas->DrawImageInt(wallpaper,
         wallpaper_cropped_rect.x(), wallpaper_cropped_rect.y(),
         wallpaper_cropped_rect.width(), wallpaper_cropped_rect.height(),