Add an h1 header for histograms page.

Histograms page includes title descriptions but it isn't reflected on the window frame, which is WAI.
Including a header information, similar to system information page will be helpful for context.

Bug: 1005549
Change-Id: Ibdbe49fb5b478be472a1ec0d3a674596ac6c5192
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879562Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Ian Barkley-Yeung <iby@chromium.org>
Commit-Queue: Joon Ahn <joonbug@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710501}

content/browser/resources/histograms/histograms_internals.html

@@ -11,6 +11,7 @@
   <script src="histograms_internals.js"></script>
   <title>Histograms</title>
 </head>
+<h1>Histograms</h1>
 <p>Stats accumulated from browser startup to previous page load; reload to get stats as of this page load.
 </p>
 <button id="refresh">Refresh</button>