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

chrome/browser/cocoa/constrained_window_mac.mm

Index: chrome/browser/cocoa/constrained_window_mac.mm
diff --git a/chrome/browser/cocoa/constrained_window_mac.mm b/chrome/browser/cocoa/constrained_window_mac.mm
index ed5cba0cd897ad76e928396b545193e8899106a9..ad090a81d32ccacef57ea32d7be972de74b6bd8a 100644
--- a/chrome/browser/cocoa/constrained_window_mac.mm
+++ b/chrome/browser/cocoa/constrained_window_mac.mm
@@ -82,7 +82,13 @@ void ConstrainedWindowMac::Realize(BrowserWindowController* controller) {
 
   // 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);
 }