WebUI: Move focus to first item in menu if focus moves from outside tab

Bug: 1164838
Change-Id: Ife2d3e970b9ab70bf1fd488f47eb9bf1bf516a4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622732Reviewed-by: dpapad <dpapad@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842709}

ui/webui/resources/cr_elements/cr_menu_selector/cr_menu_selector.js

@@ -46,9 +46,10 @@ export class CrMenuSelector extends HTMLElement {
     // If the focus was moved by keyboard and is coming in from a relatedTarget
     // that is not within this menu, move the focus to the first menu item. This
     // ensures that the first menu item is always the first focused item when
-    // focusing into the menu.
+    // focusing into the menu. A null relatedTarget means the focus was moved
+    // from outside the WebContents.
     const focusMovedWithKeyboard = this.focusOutlineManager_.visible;
-    const focusMovedFromOutside = e.relatedTarget &&
+    const focusMovedFromOutside = e.relatedTarget === null ||
         !this.contains(/** @type {!HTMLElement} */ (e.relatedTarget));
     if (focusMovedWithKeyboard && focusMovedFromOutside) {
       this.getItems_()[0].focus();