A11y - for diff mode selector

Add announcement when diff mode changed.
Inform user which mode is selected when focusing button.

Change-Id: If2b58ec90d6a4ab6399fc958476b9c43210784ad
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector_html.ts b/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector_html.ts
index 73f7426..a5bf269 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector_html.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector_html.ts
@@ -36,6 +36,7 @@
     has-tooltip=""
     class$="[[_computeSideBySideSelected(mode)]]"
     title="Side-by-side diff"
+    aria-pressed="[[isSideBySideSelected(mode)]]"
     on-click="_handleSideBySideTap"
   >
     <iron-icon icon="gr-icons:side-by-side"></iron-icon>
@@ -46,6 +47,7 @@
     has-tooltip=""
     title="Unified diff"
     class$="[[_computeUnifiedSelected(mode)]]"
+    aria-pressed="[[isUnifiedSelected(mode)]]"
     on-click="_handleUnifiedTap"
   >
     <iron-icon icon="gr-icons:unified"></iron-icon>