Use the outermost embedder WebContents in ConstrainedWindowMac.

chrome/browser/ui/cocoa/constrained_window/constrained_window_mac.mm

Index: chrome/browser/ui/cocoa/constrained_window/constrained_window_mac.mm
diff --git a/chrome/browser/ui/cocoa/constrained_window/constrained_window_mac.mm b/chrome/browser/ui/cocoa/constrained_window/constrained_window_mac.mm
index 1da08e0e54dfdf0b1575f6d5192a90a1c7b00035..0cc3fa0e98e399e9f8c684496af2539e5f18a943 100644
--- a/chrome/browser/ui/cocoa/constrained_window/constrained_window_mac.mm
+++ b/chrome/browser/ui/cocoa/constrained_window/constrained_window_mac.mm
@@ -20,12 +20,15 @@ ConstrainedWindowMac::ConstrainedWindowMac(
     id<ConstrainedWindowSheet> sheet)
     : delegate_(delegate) {
   DCHECK(sheet);
-  extensions::GuestViewBase* guest_view =
-      extensions::GuestViewBase::FromWebContents(web_contents);
-  // For embedded WebContents, use the embedder's WebContents for constrained
-  // window.
-  web_contents = guest_view && guest_view->embedder_web_contents() ?
-                    guest_view->embedder_web_contents() : web_contents;
+
+  // |web_contents| may be embedded within a chain of nested GuestViews. If it
+  // is, follow the chain of embedders to the outermost WebContents and use it.
+  while (extensions::GuestViewBase* guest_view =

[Fady Samuel, 2015/03/26 01:43:02]

Drive by comment: This pattern has started emerging in a couple of places. I
wonder if it makes sense to put it in GuestViewBase?

It could either be a member method or a static method:

guest->GetTopMostWebContents()

or 

GuestViewBase::GetTopMostWebContents(web_contents)

+             extensions::GuestViewBase::FromWebContents(web_contents)) {
+    if (!guest_view->embedder_web_contents())
+      break;
+    web_contents = guest_view->embedder_web_contents();
+  }
 
   auto manager = WebContentsModalDialogManager::FromWebContents(web_contents);
   scoped_ptr<SingleWebContentsDialogManagerCocoa> native_manager(