MacViews: Support ui::MODAL_TYPE_WINDOW with a null parent window.

ui/views/widget/native_widget_mac.mm

Index: ui/views/widget/native_widget_mac.mm
diff --git a/ui/views/widget/native_widget_mac.mm b/ui/views/widget/native_widget_mac.mm
index 8ddf80139c21577cc32d25d36eae81f0bf440151..9f26789a2f15a3048e36d78eeb32a2799a5f996a 100644
--- a/ui/views/widget/native_widget_mac.mm
+++ b/ui/views/widget/native_widget_mac.mm
@@ -96,8 +96,9 @@ BridgedNativeWidget* NativeWidgetMac::GetBridgeForNativeWindow(
 }
 
 bool NativeWidgetMac::IsWindowModalSheet() const {
-  return GetWidget()->widget_delegate()->GetModalType() ==
-         ui::MODAL_TYPE_WINDOW;
+  return bridge_ && bridge_->parent() &&
+         GetWidget()->widget_delegate()->GetModalType() ==
+             ui::MODAL_TYPE_WINDOW;
 }
 
 void NativeWidgetMac::OnWindowWillClose() {
@@ -292,8 +293,13 @@ void NativeWidgetMac::InitModalType(ui::ModalType modal_type) {
 
   // System modal windows not implemented (or used) on Mac.
   DCHECK_NE(ui::MODAL_TYPE_SYSTEM, modal_type);
-  DCHECK(bridge_->parent());
-  // Everyhing happens upon show.
+
+  // A peculiarity of the constrained window framework is that it permits a
+  // dialog of MODAL_TYPE_WINDOW to have a null parent window; falling back to
+  // a non-modal window in this case.
+  DCHECK(bridge_->parent() || modal_type == ui::MODAL_TYPE_WINDOW);
+
+  // Everything happens upon show.
 }
 
 gfx::Rect NativeWidgetMac::GetWindowBoundsInScreen() const {