Mac: Only show one per-tab sheet at a time per tab.

chrome/browser/cocoa/constrained_window_mac.mm

 // Copyright (c) 2009 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.
 
 #include "chrome/browser/cocoa/constrained_window_mac.h"
 
 #import "chrome/browser/cocoa/browser_window_controller.h"
 #include "chrome/browser/tab_contents/tab_contents.h"
 #include "chrome/browser/tab_contents/tab_contents_view.h"
 #import "third_party/GTM/AppKit/GTMWindowSheetController.h"
@@ skipping 64 matching lines @@
 
 void ConstrainedWindowMac::Realize(BrowserWindowController* controller) {
   if (controller_ != nil) {
     DCHECK(controller_ == controller);
     return;
   }
   DCHECK(controller != nil);
 
   // Remember the controller we're adding ourselves to, so that we can later
   // remove us from it.
-  controller_ = controller;
-  [controller_ attachConstrainedWindow:this];
+  if ([controller attachConstrainedWindow:this])
+    controller_ = controller;
+}
+
+void ConstrainedWindowMac::SetVisible() {
+  // Only notify the delegate that the sheet is open after the sheet appeared
+  // on screen (as opposed to when the sheet was added to the current tab's
+  // sheet queue).
   delegate_->set_sheet_open(true);
 }