fix webcomponents example

[jalexanderfox]

The ReactComponent > WebComponent > ReactComponent interop example is currently partially broken:

https://github.com/facebook/react/blob/master/examples/webcomponents/index.html

In Chrome 51.x, the custom element's createdCallback

var name = this.getAttribute('name')

expect:

name === "Jim Sproch";

actual:

name === null;

To get this to work the createdCallback function can be moved to an attachedCallback.

Pull request coming soon.

[zpao]

Fix landed