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

The Windows system automatically adds a tooltip to these buttons as the button returns HTCLOSE when nc hit tested. If we add the aura tooltip as well there are two tooltips shown.

BUG=471981

Review URL: https://codereview.chromium.org/1044043004

Cr-Commit-Position: refs/heads/master@{#324183}

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;
 }