Flags WebUI: add page title

Bug: 870763
Change-Id: Id5582daf0b9d67e3cbdc259a32863ede4bedc309
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1692340Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676111}

components/flags_ui/resources/flags.html

@@ -6,6 +6,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <link rel="stylesheet" href="chrome://resources/css/text_defaults_md.css">
 </if>
+<title>Flags</title>
 
 <link rel="stylesheet" href="flags.css">