Revert of Remove NPAPI plugin from chromoting webapp.

remoting/webapp/host_screen.js

Index: remoting/webapp/host_screen.js
diff --git a/remoting/webapp/host_screen.js b/remoting/webapp/host_screen.js
index bfa3827b7d1f09f255857916c270cfecdcff5e26..d1a26fcb11de2e13684ea61ebe94229c2139f398 100644
--- a/remoting/webapp/host_screen.js
+++ b/remoting/webapp/host_screen.js
@@ -33,13 +33,25 @@
   var hostInstallDialog = null;
 
   var tryInitializeDispatcher = function() {
-    hostDispatcher.initialize(onDispatcherInitialized,
+    hostDispatcher.initialize(createPluginForIt2Me,
+                              onDispatcherInitialized,
                               onDispatcherInitializationFailed);
   }
 
+  /** @return {remoting.HostPlugin} */
+  var createPluginForIt2Me = function() {
+    return remoting.createNpapiPlugin(
+        document.getElementById('host-plugin-container'));
+  }
+
   var onDispatcherInitialized = function () {
-    // Host already installed.
-    remoting.startHostUsingDispatcher_(hostDispatcher);
+    if (hostDispatcher.usingNpapi()) {
+      hostInstallDialog = new remoting.HostInstallDialog();
+      hostInstallDialog.show(tryInitializeDispatcher, onInstallError);
+    } else {
+      // Host alrady installed.
+      remoting.startHostUsingDispatcher_(hostDispatcher);
+    }
   };
 
   /** @param {remoting.Error} error */
@@ -169,6 +181,8 @@
         remoting.setMode(remoting.AppMode.HOST_SHARE_FINISHED);
       }
     }
+    remoting.hostSession.cleanup();
+
   } else if (state == remoting.HostSession.State.ERROR) {
     console.error('Host state: ERROR');
     showShareError_(remoting.Error.UNEXPECTED);