Use CSS box-shadow rather than border to highlight selected gate chip. (#2856)

* Use CSS outline rather than border to highlight selected gate chip.

* Use box-shadow to work-around a bug in safari

client-src/elements/chromedash-gate-chip.js

@@ -58,7 +58,7 @@ class ChromedashGateChip extends LitElement {
      }
 
      sl-button.selected::part(base) {
-       border: 2px solid var(--dark-spot-color);
+       box-shadow: 0 0 0 2px var(--dark-spot-color);
      }
 
      sl-button::part(base):hover {