[AppRemoting] Break out AppRemoting shared module (re-land).

remoting/webapp/app_remoting/manifest_common.json.jinja2

   {{ MANIFEST_KEY_FOR_UNOFFICIAL_BUILD }}
   "name": "{{APP_NAME}}",
   "description": "{{APP_DESCRIPTION}}",
   "version": "{{ FULL_APP_VERSION }}",
   "manifest_version": 2,
   "default_locale": "en",
   "app": {
     "background": {
-      "scripts": ["ar_background.js", "platform.js"]
+      "page": "arv_background.html"
     }
   },
+  "import": [{
+      "id": "koejkfhmphamcgafjmkellhnekdkopod"
+    }
+  ],
   "icons": {
     "128": "icon128.png",
     "48": "icon48.png",
     "16": "icon16.png"
   },
   "optional_permissions": [
     "<all_urls>"
   ],
   "permissions": [
     "{{ APP_REMOTING_API_BASE_URL }}/*",
@@ skipping 30 matching lines @@
     "client_id": "{{ REMOTING_IDENTITY_API_CLIENT_ID }}",
     "scopes": [
       {{ OAUTH_GDRIVE_SCOPE }}
       "https://www.googleapis.com/auth/appremoting.runapplication",
       "https://www.googleapis.com/auth/googletalk",
       "https://www.googleapis.com/auth/userinfo#email",
       "https://www.googleapis.com/auth/userinfo.profile"
     ]
   },
   "sandbox": {
-    "pages": [ "wcs_sandbox.html" ]
+    "pages": [ "arv_wcs_sandbox.html" ]
   }