Added headings to the extensions page

Helps screen reader users navigate extensions page easily by jumping from heading to heading.
As a result of direct feedback from a screen reader user.
R=rdevlin.cronin@chromium.org, scottchen@chromium.org
Bug: 827264

Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I7a7a3ad3c9ea6e57d5d3eaa73a5ee9fccd35fd2a
Reviewed-on: https://chromium-review.googlesource.com/981902Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547395}

chrome/browser/resources/md_extensions/item.html

@@ -251,7 +251,8 @@
               doesn't shrink (because it's not display: flex).-->
           <div>
             <div id="name-and-version" class="layout horizontal center">
-              <div id="name" class="clippable-flex-text">[[data.name]]</div>
+              <div id="name" role="heading" aria-level="3"
+                  class="clippable-flex-text">[[data.name]]</div>
               <span id="version" hidden$="[[!inDevMode]]">
                 [[data.version]]
               </span>