Bookmarks and History WebUI: Make screen reader read out selected count

Fixed: 1142080
Change-Id: Ib9fee0ddff34897d56779009ba7463a12fb57940
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2580183
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836232}

ui/webui/resources/cr_elements/cr_toolbar/BUILD.gn

@@ -20,6 +20,7 @@ js_library("cr_toolbar_search_field") {
 }
 
 js_library("cr_toolbar_selection_overlay") {
+  deps = [ "//third_party/polymer/v1_0/components-chromium/iron-a11y-announcer:iron-a11y-announcer-extracted" ]
 }
 
 js_library("cr_toolbar") {
@@ -86,6 +87,7 @@ js_library("cr_toolbar_search_field.m") {
 js_library("cr_toolbar_selection_overlay.m") {
   sources = [ "$root_gen_dir/ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar_selection_overlay.m.js" ]
   deps = [
+    "//third_party/polymer/v3_0/components-chromium/iron-a11y-announcer:iron-a11y-announcer",
     "//third_party/polymer/v3_0/components-chromium/polymer:polymer_bundled",
   ]
   extra_deps = [ ":cr_toolbar_selection_overlay_module" ]

ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar_selection_overlay.html

 <link rel="import" href="../../html/polymer.html">
 
+<link rel="import" href="chrome://resources/polymer/v1_0/iron-a11y-announcer/iron-a11y-announcer.html">
 <link rel="import" href="../cr_button/cr_button.html">
 <link rel="import" href="../cr_icon_button/cr_icon_button.html">
 <link rel="import" href="../icons.html">

ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar_selection_overlay.js

@@ -69,6 +69,9 @@ Polymer({
       this.selectionLabel_ =
           this.show ? this.selectionLabel : this.selectionLabel_;
       this.setAttribute('aria-label', this.selectionLabel_);
+
+      Polymer.IronA11yAnnouncer.requestAvailability();
+      this.fire('iron-announce', {text: this.selectionLabel});
     });
   },