Remove nodeIntegration warning of webview in BrowserWindow

nodeintegration attribute is ignored in React because it's unknown.
So fortunately the integration is not working.
New 'will-attach-webview' allows us to set nodeIntegration = false.

src/browser/components/MattermostView.jsx

@@ -233,7 +233,6 @@ const MattermostView = React.createClass({
           preload={preloadJS}
           src={this.props.src}
           ref='webview'
-          nodeintegration='false'
         />
       </div>);
   }

src/main.js

@@ -466,6 +466,10 @@ app.on('ready', () => {
     console.log('The application has crashed.');
   });
 
+  mainWindow.webContents.on('will-attach-webview', (event, webPreferences) => {
+    webPreferences.nodeIntegration = false;
+  });
+
   mainWindow.on('unresponsive', () => {
     console.log('The application has become unresponsive.');
   });

test/specs/security_test.js

@@ -51,17 +51,18 @@ describe('application', function desc() {
 
   it('should NOT be able to call Node.js API in webview', () => {
     env.addClientCommands(this.app.client);
+
+    // webview is handled as a window by chromedriver.
     return this.app.client.
+      windowByIndex(1).isNodeEnabled().should.eventually.be.false.
+      windowByIndex(2).isNodeEnabled().should.eventually.be.false.
+      windowByIndex(0).
       getAttribute('webview', 'nodeintegration').then((nodeintegration) => {
         // nodeintegration is an array of string
         nodeintegration.forEach((n) => {
           n.should.equal('false');
         });
-      }).
-
-      // webview is handled as a window by chromedriver.
-      windowByIndex(1).isNodeEnabled().should.eventually.be.false.
-      windowByIndex(2).isNodeEnabled().should.eventually.be.false;
+      });
   });
 
   it('should NOT be able to call Node.js API in a new window', () => {