history: tag card-title entries as ARIA headings

This makes them visible as heading when using heading navigation in
assistive technology. They are logically headings, but are using div
instead of h2 and I am somewhat afraid to make them h2 :)

Bug: 957368
Change-Id: I5978a49dfa8a4c2725fd491228e5eee9e01e032d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1604348
Auto-Submit: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659027}

chrome/browser/resources/history/history_item.html

@@ -172,7 +172,7 @@
       <div id="background-clip">
         <div id="background"></div>
       </div>
-      <div id="date-accessed" class="card-title">
+      <div id="date-accessed" class="card-title" role="heading" aria-level="2">
         [[cardTitle_(numberOfItems, item.dateRelativeDay, searchTerm)]]
       </div>
       <div id="item-container" focus-row-container