mac: attempt to fix a startup perf regression after r174653

chrome/browser/ui/cocoa/image_button_cell.h

 // Copyright (c) 2011 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 CHROME_BROWSER_UI_COCOA_IMAGE_BUTTON_CELL_H_
 #define CHROME_BROWSER_UI_COCOA_IMAGE_BUTTON_CELL_H_
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/memory/scoped_nsobject.h"
@@ skipping 18 matching lines @@
 @optional
 // Sent from an ImageButtonCell to its view when the mouse enters or exits the
 // cell.
 - (void)mouseInsideStateDidChange:(BOOL)isInside;
 @end
 
 // A button cell that can disable a different image for each possible button
 // state. Images are specified by image IDs.
 @interface ImageButtonCell : NSButtonCell {
  @private
-  scoped_nsobject<NSImage> image_[image_button_cell::kButtonStateCount];
+  struct {
+    // At most one of these two fields will be non-null.
+    int imageId;
+    scoped_nsobject<NSImage> image;
+  } image_[image_button_cell::kButtonStateCount];
   NSInteger overlayImageID_;
   BOOL isMouseInside_;
 }
 
 @property(assign, nonatomic) NSInteger overlayImageID;
 @property(assign, nonatomic) BOOL isMouseInside;
 
 // Sets the image for the given button state using an image ID.
-// The image will be loaded from a resource pak.
+// The image will be lazy loaded from a resource pak -- important because
+// this is in the hot path for startup.
 - (void)setImageID:(NSInteger)imageID
     forButtonState:(image_button_cell::ButtonState)state;
 
 // Sets the image for the given button state using an image.
 - (void)setImage:(NSImage*)image
   forButtonState:(image_button_cell::ButtonState)state;
 
 @end
 
 #endif // CHROME_BROWSER_UI_COCOA_IMAGE_BUTTON_CELL_H_