Add ARIA alerts to cr-input error messages

-In error input scenarios, current implementation will display useful
 error text but is not read out by screen readers. This change forces
 these alerts to be read by screen readers.

Bug: 1012532
Change-Id: Iadd263d17bf648660f6e365495093543cab86666
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1854529Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Commit-Queue: Gavin Williams <gavinwill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706199}

ui/webui/resources/cr_elements/cr_input/cr_input.html

@@ -130,7 +130,7 @@
       </div>
       <slot name="suffix"></slot>
     </div>
-    <div id="error">[[errorMessage]]</div>
+    <div id="error" role="alert">[[errorMessage]]</div>
   </template>
   <script src="cr_input.js"></script>
 </dom-module>