follow_hyperlink is broken when targeting a browsing context for another process

[jdm]

It chooses a browsing context, then tries to get the window from it so it can call load_url. This is broken if the window exists in another process; we need to go through the constellation in that case.

[jdm]

This is also true about form submission and area element hyperlinks.

[jdm]

One way to make this issue more clear would be to rename the WindowProxy::document method to WindowProxy::document_in_current_process, and possibly have a return value that differentiates between "there is no record of the document" and "this document is not in this process".