mac: attempt to fix a startup perf regression after r174653

chrome/browser/ui/cocoa/image_button_cell.h

Index: chrome/browser/ui/cocoa/image_button_cell.h
diff --git a/chrome/browser/ui/cocoa/image_button_cell.h b/chrome/browser/ui/cocoa/image_button_cell.h
index 8d64c1e97f3d1f1cd93606ad1dea6d2bd9cd2ce0..74feb94c36e0fce1b58b3fd322dcf77de3926513 100644
--- a/chrome/browser/ui/cocoa/image_button_cell.h
+++ b/chrome/browser/ui/cocoa/image_button_cell.h
@@ -36,7 +36,11 @@ enum ButtonState {
 // 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_;
 }
@@ -45,7 +49,8 @@ enum ButtonState {
 @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;