[OSX] Allow one zoom bubble to override another.

chrome/browser/ui/cocoa/browser/zoom_bubble_controller.mm

Index: chrome/browser/ui/cocoa/browser/zoom_bubble_controller.mm
diff --git a/chrome/browser/ui/cocoa/browser/zoom_bubble_controller.mm b/chrome/browser/ui/cocoa/browser/zoom_bubble_controller.mm
index 234eb6d92827d1b22738ade1fe3a7921cbfe6ddc..05c9221a6f3caa69d83dc0136eb528dd24f0a3b9 100644
--- a/chrome/browser/ui/cocoa/browser/zoom_bubble_controller.mm
+++ b/chrome/browser/ui/cocoa/browser/zoom_bubble_controller.mm
@@ -75,6 +75,8 @@ void SetZoomBubbleAutoCloseDelayForTesting(NSTimeInterval time_interval) {
 
 @implementation ZoomBubbleController
 
+@synthesize delegate = delegate_;
+
 - (id)initWithParentWindow:(NSWindow*)parentWindow
                   delegate:(ZoomBubbleControllerDelegate*)delegate {
   base::scoped_nsobject<InfoBubbleWindow> window(
@@ -126,6 +128,10 @@ void SetZoomBubbleAutoCloseDelayForTesting(NSTimeInterval time_interval) {
 }
 
 - (void)onZoomChanged {
+  // |delegate_| may be set null by this object's owner.
+  if (!delegate_)
+    return;
+
   // TODO(shess): It may be appropriate to close the window if
   // |contents| or |zoomController| are NULL.  But they can be NULL in
   // tests.
@@ -169,8 +175,11 @@ void SetZoomBubbleAutoCloseDelayForTesting(NSTimeInterval time_interval) {
 }
 
 - (void)windowWillClose:(NSNotification*)notification {
-  delegate_->OnClose();
-  delegate_ = NULL;
+  // |delegate_| may be set null by this object's owner.
+  if (delegate_) {
+    delegate_->OnClose();
+    delegate_ = NULL;
+  }
   [NSObject cancelPreviousPerformRequestsWithTarget:self
                                            selector:@selector(autoCloseBubble)
                                              object:nil];