[GENERIC viewer] Try to improve a11y, for search results, in the findbar (issue 14525)

The following MDN articles may be helpful here:
 - https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes/aria-invalid
 - https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes/aria-live

Author: Snuffleupagus

web/pdf_find_bar.js

@@ -127,6 +127,7 @@ class PDFFindBar {
         break;
     }
     this.findField.setAttribute("data-status", status);
+    this.findField.setAttribute("aria-invalid", state === FindState.NOT_FOUND);
 
     findMsg.then(msg => {
       this.findMsg.textContent = msg;

web/viewer.html

@@ -116,7 +116,7 @@
       <div id="mainContainer">
         <div class="findbar hidden doorHanger" id="findbar">
           <div id="findbarInputContainer">
-            <input id="findInput" class="toolbarField" title="Find" placeholder="Find in document…" tabindex="91" data-l10n-id="find_input">
+            <input id="findInput" class="toolbarField" title="Find" placeholder="Find in document…" tabindex="91" data-l10n-id="find_input" aria-invalid="false">
             <div class="splitToolbarButton">
               <button id="findPrevious" class="toolbarButton findPrevious" title="Find the previous occurrence of the phrase" tabindex="92" data-l10n-id="find_previous">
                 <span data-l10n-id="find_previous_label">Previous</span>
@@ -141,10 +141,10 @@
             <label for="findEntireWord" class="toolbarLabel" data-l10n-id="find_entire_word_label">Whole Words</label>
           </div>
 
-          <div class="findbarMessageContainer">
+          <div class="findbarMessageContainer" aria-live="polite">
             <span id="findResultsCount" class="toolbarLabel"></span>
           </div>
-          <div class="findbarMessageContainer">
+          <div class="findbarMessageContainer" aria-live="assertive">
             <span id="findMsg" class="toolbarLabel"></span>
           </div>
         </div>  <!-- findbar -->