Cocoa: Support constrained windows for platform apps

chrome/browser/ui/cocoa/constrained_window_mac.mm

Index: chrome/browser/ui/cocoa/constrained_window_mac.mm
diff --git a/chrome/browser/ui/cocoa/constrained_window_mac.mm b/chrome/browser/ui/cocoa/constrained_window_mac.mm
index 1d018f160838b2dc24cbd176e3f4a787908aaef2..d826dc3d0b36054ec1ba316c04d44a64d1ff3030 100644
--- a/chrome/browser/ui/cocoa/constrained_window_mac.mm
+++ b/chrome/browser/ui/cocoa/constrained_window_mac.mm
@@ -4,7 +4,6 @@
 
 #include "chrome/browser/ui/cocoa/constrained_window_mac.h"
 
-#import "chrome/browser/ui/cocoa/browser_window_controller.h"
 #include "chrome/browser/ui/constrained_window_tab_helper.h"
 #include "chrome/browser/ui/tab_contents/tab_contents.h"
 #include "content/public/browser/web_contents.h"
@@ -105,12 +104,26 @@ void ConstrainedWindowMac::ShowConstrainedWindow() {
   // The WebContents only has a native window if it is currently visible. In
   // this case, open the sheet now. Else, Realize() will be called later, when
   // our tab becomes visible.
-  NSWindow* browserWindow =
+  NSWindow* window =
       tab_contents_->web_contents()->GetView()->GetTopLevelNativeWindow();
-  BrowserWindowController* browser_controller =
-      [BrowserWindowController browserWindowControllerForWindow:browserWindow];
-  if ([browser_controller canAttachConstrainedWindow])
-    Realize(browser_controller);
+  NSWindowController<ConstrainedWindowSupport>* window_controller = nil;
+  while (window) {
+    if ([[window windowController] conformsToProtocol:
+            @protocol(ConstrainedWindowSupport)]) {
+      window_controller = [window windowController];
+      break;
+    }
+    window = [window parentWindow];
+  }
+
+  // It's valid for the window to be nil. For example, background tabs don't
+  // have a window set. However, if a window exists then there should always
+  // be a window controller that implements the ConstrainedWindowSupport
+  // protocol.
+  DCHECK(!window || window_controller);
+
+  if ([window_controller canAttachConstrainedWindow])
+    Realize(window_controller);
 }
 
 void ConstrainedWindowMac::CloseConstrainedWindow() {
@@ -131,7 +144,8 @@ void ConstrainedWindowMac::CloseConstrainedWindow() {
   delete this;
 }
 
-void ConstrainedWindowMac::Realize(BrowserWindowController* controller) {
+void ConstrainedWindowMac::Realize(
+    NSWindowController<ConstrainedWindowSupport>* controller) {
   if (!should_be_visible_)
     return;