InspectableWebContents should be able to accept existing WebContents.

When implementing `window.open`, we need to override `content::WebContentsDelegate::WebContentsCreated` which already created a `WebContents` and we should wrap it around an existing window instead of creating a new `WebContents`.

So `InspectableWebContents` should accept a `WebContents` as parameter, instead of always creating a new one.