Factor out uses of the WebContentsModalDialog interface from platform-independent code

chrome/browser/ui/cocoa/constrained_web_dialog_delegate_mac.mm

Index: chrome/browser/ui/cocoa/constrained_web_dialog_delegate_mac.mm
diff --git a/chrome/browser/ui/cocoa/constrained_web_dialog_delegate_mac.mm b/chrome/browser/ui/cocoa/constrained_web_dialog_delegate_mac.mm
index dd710c11266064362b6467290971d5fef6ba0d90..055ca8b3a9dd67358e3ab7a1906c20e3df0305fc 100644
--- a/chrome/browser/ui/cocoa/constrained_web_dialog_delegate_mac.mm
+++ b/chrome/browser/ui/cocoa/constrained_web_dialog_delegate_mac.mm
@@ -46,8 +46,8 @@ class ConstrainedWebDialogDelegateMac :
   virtual void ReleaseWebContentsOnDialogClose() OVERRIDE {
     return impl_->ReleaseWebContentsOnDialogClose();
   }
-  virtual WebContentsModalDialog* GetWindow() OVERRIDE {
-    return impl_->GetWindow();
+  virtual gfx::NativeWindow GetNativeWindow() OVERRIDE {
+    return impl_->GetNativeWindow();
   }
   virtual WebContents* GetWebContents() OVERRIDE {
     return impl_->GetWebContents();
@@ -92,7 +92,6 @@ ConstrainedWebDialogDelegateMac::ConstrainedWebDialogDelegateMac(
           initWithCustomWindow:window_]);
   constrained_window_.reset(new ConstrainedWindowMac(
       this, web_contents, sheet));
-  return impl_->set_window(constrained_window_.get());
 }
 
 ConstrainedWebDialogDelegate* CreateConstrainedWebDialog(