This is needed to handle createWindow where a WebContentsAdapter will
be created to wrap the new WebContents and will passed up through the
API. If the application doesn't handle the callback, this will allow
managing the lifetime of the WebContents more easily.

Change-Id: I40ae1973a9bdf8c3d75f9ff137228d48ed92cfb2
Reviewed-by: Andras Becsi <andras.becsi@digia.com>