Don't add an aura tooltip to bubble close buttons on Windows.

ui/views/bubble/bubble_frame_view.cc

Index: ui/views/bubble/bubble_frame_view.cc
diff --git a/ui/views/bubble/bubble_frame_view.cc b/ui/views/bubble/bubble_frame_view.cc
index dff1e993c38982f0e6869eebfe6b7b7a9b7b85c6..cd616ebb526f3b4c56d0fa3b374bf641f37e5fdb 100644
--- a/ui/views/bubble/bubble_frame_view.cc
+++ b/ui/views/bubble/bubble_frame_view.cc
@@ -105,7 +105,11 @@ LabelButton* BubbleFrameView::CreateCloseButton(ButtonListener* listener) {
                   *rb.GetImageNamed(IDR_CLOSE_DIALOG_P).ToImageSkia());
   close->SetBorder(nullptr);
   close->SetSize(close->GetPreferredSize());
+#if !defined(OS_WIN)
+  // Windows will automatically create a tooltip for the close button based on
+  // the HTCLOSE result from NonClientHitTest().
   close->SetTooltipText(l10n_util::GetStringUTF16(IDS_APP_CLOSE));
+#endif
   return close;
 }