downloads/bookmarks: change <h2> to <h1> for better screen reader semantics.

chrome/browser/resources/search_header.css

 /* Copyright 2014 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
 /* This is a common file for pages that have a title and search box as their
  * header. This visually looks like:
  * _____________________________________________________________________________
  * |                                                                           |
  * |  I Am So Great!                                  [ Search greatness... ]  |
  * |___________________________________________________________________________|
  * | Summary (90235 things currently great)                      _Great links_ |
  *
  * The title expands to fill any space it needs and the search bar is locked to
  * the end of the header (e.g., right in LTR, left in RTL).
  */
 
 header,
 .summary {
   min-width: 400px;
 }
 
 header {
   align-items: center;
   display: flex;
   padding: 0 12px;
 }
 
-header h2 {
+header h1 {
   flex: 1;
   /* TODO(dbeam): reconcile font/line-height/margin with chrome_shared.css */
   font-size: 1.5em;
   font-weight: normal;
   line-height: 1;
   margin: 15px 0;
 }
 
 header > :-webkit-any(form, input[type='search']) {
   flex: none;
@@ skipping 10 matching lines @@
 .summary > * {
   flex: none;
 }
 
 .summary :first-child {
   flex: 1;
   overflow: hidden;
   text-overflow: ellipsis;
   white-space: nowrap;
 }