Enforce that only one constrained window is displayed at once per tab.

chrome/browser/tab_contents/tab_contents.cc

Index: chrome/browser/tab_contents/tab_contents.cc
diff --git a/chrome/browser/tab_contents/tab_contents.cc b/chrome/browser/tab_contents/tab_contents.cc
index 0d2ba960bdf1286b2d74be3f88f1df38e7468d37..16c2637ef9e129b535f593b0352cbaa65d19a116 100644
--- a/chrome/browser/tab_contents/tab_contents.cc
+++ b/chrome/browser/tab_contents/tab_contents.cc
@@ -824,6 +824,10 @@ ConstrainedWindow* TabContents::CreateConstrainedDialog(
   ConstrainedWindow* window =
       ConstrainedWindow::CreateConstrainedDialog(this, delegate);
   child_windows_.push_back(window);
+
+  if (child_windows_.size() == 1)
+    window->ShowConstrainedWindow();
+
   return window;
 }
 
@@ -1054,8 +1058,13 @@ void TabContents::OnStartDownload(DownloadItem* download) {
 void TabContents::WillClose(ConstrainedWindow* window) {
   ConstrainedWindowList::iterator it =
       find(child_windows_.begin(), child_windows_.end(), window);
+
+  bool removed_topmost_window = it == child_windows_.begin();
   if (it != child_windows_.end())
     child_windows_.erase(it);
+
+  if (removed_topmost_window && child_windows_.size() > 0)
+    child_windows_[0]->ShowConstrainedWindow();
 }
 
 void TabContents::WillCloseBlockedPopupContainer(