Remove the BubbleFrameView close button from tab traversal on all platforms.

The button defaults to FocusBehavior::ACCESSIBLE_ONLY which (apart from
on Mac) only affects buttons in AccessiblePaneViews (which the close
button is not). Make it FocusBehavior::NEVER so the behaviour is
consistent across platforms.

Note this does not affect screen readers' ability to focus the element.
Keyboard access to this element when not using a screen reader is done
via the ESC key handler in DialogClientView.

BUG=741251

Review-Url: https://codereview.chromium.org/2982533002
Cr-Commit-Position: refs/heads/master@{#486228}

ui/views/bubble/bubble_frame_view.cc

@@ -133,6 +133,12 @@ Button* BubbleFrameView::CreateCloseButton(ButtonListener* listener) {
   }
   close_button->SetTooltipText(l10n_util::GetStringUTF16(IDS_APP_CLOSE));
   close_button->SizeToPreferredSize();
+
+  // Remove the close button from tab traversal on all platforms. Note this does
+  // not affect screen readers' ability to focus the close button. Keyboard
+  // access to the close button when not using a screen reader is done via the
+  // ESC key handler in DialogClientView.
+  close_button->SetFocusBehavior(View::FocusBehavior::NEVER);
   return close_button;
 }