UserAgent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/66.0.3359.170 Safari/537.36

Steps to reproduce the problem:
1. Created a javascript application using hterm
2. launched hterm via application
3. press keyboard function key f1

What is the expected behavior?
 when function f1 is pressed it should call handleKey as shown below. 
  t = new hterm.Terminal();
  io.onVTKeystroke = function(str) {
      handleKey(str);
      };

What went wrong?
f1 is popping up windows help in internet explorer in addition to calling handleKey.  

Did this work before? No 

Chrome version: 66.0.3359.170  Channel: n/a
OS Version: 6.1 (Windows 7, Windows Server 2008 R2)
Flash Version: 

Internet explorer fires the windows help event whenever the function key f1 is pressed.  this has to be overridden in the iframe generated by hterm.   

Would need to add this code in hterm.ScrollPort.prototype.decorate so that document.onhelp is disabled in the iframe.

doc.onhelp = function() {
    return false;  
    };  

 This will allows f1 to be handled by hterm without windows help popping up.