Notify screen reader search results in "chrome://flags" page

When a user is searching for a flag and there is no matching results found,
a message saying "No matching experiments" is displayed. This PR adds ability
for screen readers to announce this message.

Bug: 983365
Change-Id: I3fc458a105cefb9294954d1c4d6a91f5f77e0212
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1698993Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Commit-Queue: Andrii Sagaidak <ansagaid@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#676933}

components/flags_ui/resources/flags.html

@@ -175,7 +175,7 @@
               </div>
             </div>
           </div>
-          <div class="no-match hidden">No matching experiments</div>
+          <div class="no-match hidden" role="alert">No matching experiments</div>
         </div>
       </li>
 <!-- Unsupported experiments are not shown on iOS -->
@@ -206,7 +206,7 @@
               <div class="flex experiment-actions">Not available on your platform.</div>
             </div>
           </div>
-          <div class="no-match hidden">
+          <div class="no-match hidden" role="alert">
             No matching experiments
           </div>
         </div>