CLC-4374 Use Canvas's tool_inline support to resize LTI iframe

https://jira.ets.berkeley.edu/jira/browse/CLC-4374

Instructure now provides built-in support for eliminating nested scrollbars in LTI tools, and complains in the JavaScript console if customers continue to use their own event listeners.

Also added comments for the remaining bits of our event handler.
@raydavis indeed and your comment makes total sense. However, we should not have an empty `catch` block in our JS code. Would it be possible to add the following to the `catch` block?

```javascript
if (window.console && console.log) {
    console.log('Canvas Customization - onmessage', err);
}
```

That way we would immediately see the message in the console without actually causing the JS to stop executing.