Include device_sync.mojom-lite.js on proximity-auth debug page

This fixes the console error on chrome://proximity-auth, which reads,

  Uncaught TypeError: Cannot read property 'mojom' of undefined
      at multidevice_setup.mojom-lite.js:1715

This refers to instances of
chromeos.device_sync.mojom.ConnectivityStatus.

Bug: 1025053
Change-Id: I1ddbe24f8354b08c0cf3c33cd6e402f4b920b78b
Fixed: 1025053
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1919567Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Josh Nohle <nohle@chromium.org>
Auto-Submit: Josh Nohle <nohle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715809}

chromeos/components/multidevice/debug_webui/resources/proximity_auth.html

@@ -17,6 +17,8 @@
   </script>
   <script src="chrome://resources/mojo/chromeos/components/multidevice/mojom/multidevice_types.mojom-lite.js">
   </script>
+  <script src="chrome://resources/mojo/chromeos/services/device_sync/public/mojom/device_sync.mojom-lite.js">
+  </script>
   <script src="chrome://resources/mojo/chromeos/services/multidevice_setup/public/mojom/multidevice_setup.mojom-lite.js">
   </script>
 </head>