fix(ui): Fix cross-browser focus outline

It turns out that `-webkit-focus-ring-color` is no longer working
across platforms in Chrome.  It works on Mac, but not anywhere else.

Removing it creates a consistent focus outline across browsers and
platforms.  It was only being used on Safari and Chrome anyway,
neither of which seem to need it.

Closes shaka-project#2863

Change-Id: Ic6df1a10f14a0907558595e53bef80ac349e6b9d

ui/less/buttons.less

@@ -92,10 +92,6 @@
     /* Most browsers will fall back to "Highlight" (system setting) color for
      * the focus outline. */
     outline: 1px solid Highlight;
-
-    /* WebKit-based and Blink-based browsers have this as their default outline
-     * color. */
-    outline: 1px solid -webkit-focus-ring-color;
   }
 
   /* Disable this Mozilla-specific focus ring, since we have an outline defined