Centre message windows when they are shown.

This adds a missing optional parameter to resizeWindowToContent. The parameter
is already specified when the message is updated--not including it here too was
an oversight.

Review URL: https://codereview.chromium.org/1133873002

Cr-Commit-Position: refs/heads/master@{#329202}

remoting/webapp/base/js/message_window.js

@@ -130,7 +130,7 @@ MessageWindowImpl.prototype.onMessage_ = function(event) {
       chrome.app.window.current().onClosed.addListener(
           this.sendReply_.bind(this, event.source, messageId, 0));
 
-      base.resizeWindowToContent();
+      base.resizeWindowToContent(true);
       chrome.app.window.current().show();
       break;