Add WidgetTest::GetAllWidgets() to find dialogs created by TestBrowserDialog.

ui/views/test/widget_test_mac.mm

Index: ui/views/test/widget_test_mac.mm
diff --git a/ui/views/test/widget_test_mac.mm b/ui/views/test/widget_test_mac.mm
index 5b3f9e5bb3b849a8569ca8e2abac946dc9345312..2f14759dade34a82a0fa1ac3f1bf58224bacb5f8 100644
--- a/ui/views/test/widget_test_mac.mm
+++ b/ui/views/test/widget_test_mac.mm
@@ -85,5 +85,15 @@ bool WidgetTest::IsNativeWindowTransparent(gfx::NativeWindow window) {
   return ![window isOpaque];
 }
 
+// static
+Widget::Widgets WidgetTest::GetAllWidgets() {
+  Widget::Widgets all_widgets;
+  for (NSWindow* window : [NSApp windows]) {
+    if (Widget* widget = Widget::GetWidgetForNativeWindow(window))
+      all_widgets.insert(widget);
+  }
+  return all_widgets;
+}
+
 }  // namespace test
 }  // namespace views