Support adding the Webview in a ScrolledWindow

[nacho]

We need to extend the webview to properly override the measure methods so we can provide a minimum and natural size for the webview. The problem is that today servo does not provide an api to get the page size. jdm provided the following comment:

It's a reasonable request; you can probably approximate it right now by calling webview.evaluate_js and evaluating
document.body.scrollHeight.

[bilelmoussaoui]

You should implement Scrollable on the Webview & then the user can embed that widget inside a ScrolledWindow