Fix JSCompile.

Review URL: https://codereview.chromium.org/571463003

Cr-Commit-Position: refs/heads/master@{#295211}

remoting/webapp/background/background.js

@@ -61,7 +61,7 @@ function initializeBackgroundService(appLauncher) {
 
 function main() {
   if (base.isAppsV2()) {
-    appLauncher = new remoting.V2AppLauncher();
+    var appLauncher = new remoting.V2AppLauncher();
     initializeAppV2(appLauncher);
   }
 }