Enforce that only one constrained window is displayed at once 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..5670dabc8dad5f352efdbc9481b88686a077d4ff 100644
--- a/chrome/browser/cocoa/constrained_window_mac.mm
+++ b/chrome/browser/cocoa/constrained_window_mac.mm
@@ -46,9 +46,16 @@ ConstrainedWindowMac::ConstrainedWindowMac(
     TabContents* owner, ConstrainedWindowMacDelegate* delegate)
     : owner_(owner),
       delegate_(delegate),
-      controller_(nil) {
+      controller_(nil),
+      should_be_visible_(false) {
   DCHECK(owner);
   DCHECK(delegate);
+}
+
+ConstrainedWindowMac::~ConstrainedWindowMac() {}
+
+void ConstrainedWindowMac::ShowConstrainedWindow() {
+  should_be_visible_ = true;
 
   // The TabContents only has a native window if it is currently visible. In
   // this case, open the sheet now. Else, Realize() will be called later, when
@@ -61,8 +68,6 @@ ConstrainedWindowMac::ConstrainedWindowMac(
   }
 }
 
-ConstrainedWindowMac::~ConstrainedWindowMac() {}
-
 void ConstrainedWindowMac::CloseConstrainedWindow() {
   // Note: controller_ can be `nil` here if the sheet was never realized. That's
   // ok.
@@ -74,6 +79,9 @@ void ConstrainedWindowMac::CloseConstrainedWindow() {
 }
 
 void ConstrainedWindowMac::Realize(BrowserWindowController* controller) {
+  if (!should_be_visible_)
+    return;
+
   if (controller_ != nil) {
     DCHECK(controller_ == controller);
     return;