Move the loading of documents in iframes into HTMLIFrameElement.

Right now, the load is kicked off inside the parser glue. This is unfortunate
for several reasons:

1) we'd like to replace the current parser (libhubbub) by our own parser,
   written in Rust, so code intertwined with the parser will have to be
   rewritten;
2) it is impossible to support dynamically (i.e. from script) created iframes
   in this way;
3) the code flow around loading subdocuments is complicated needlessly.

This commit adds the constellation channel (on which the message to actually
load the document is sent) as a field on the Page, to allow HTMLIFrameElement
to access it.

In rewriting the code, support for dynamically created iframes is added, and
a task failure is avoided when the value of the src attribute can not be
parsed.