CrOS Media App: Update app.html script import order.

With cl/343193875 app_main.js reads window.loadTimeData. Re-order
our imports so window.loadTimeData always exists before we try
read it (see the chat thread from
https://buganizer.corp.google.com/issues/172985712#comment5)

Note: this needs to land before cl/343193875.

Bug: b/172985712
Change-Id: I884426062c255470a226d75a2d479ce858a2c11d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2548582Reviewed-by: David Lei <dlei@google.com>
Reviewed-by: Rachel Carpenter <carpenterr@chromium.org>
Commit-Queue: David Lei <dlei@google.com>
Commit-Queue: Rachel Carpenter <carpenterr@chromium.org>
Auto-Submit: David Lei <dlei@google.com>
Cr-Commit-Position: refs/heads/master@{#829076}

chromeos/components/media_app_ui/resources/app.html

@@ -11,11 +11,14 @@
   }
 </style>
 
-<script src="/js/app_main.js"></script>
+<!-- Order is important for these files since "app_main.js" reads from 
+	`window.loadTimeData`. -->
 <script src="/media_app_app_scripts.js"></script>
 
 <!-- Populates `window.loadTimeData`, needs to be after
   "media_app_app_scripts.js" which loads "load_time_data.js" -->
 <script src="/strings.js"></script>
 
+<script src="/js/app_main.js"></script>
+
 </html>