Make load event comply with the spec

[jdm]

http://www.whatwg.org/specs/web-apps/current-work/multipage/the-end.html#the-end

We need the ability to track resource loads that are fired during the initial parse. It likely makes sense to do this in concert with the BrowserContext implementation (#1417).

[jdm]

<jgraham> It would be nice if it worked in such a way that you could always tell what was blocking the load event to help debugging NSL (never stops loading) type bugs

[jdm]

• Ensure scripts execute in order encountered during parsing
• Dispatch DOMContentLoaded once scripts have fired
• Add a load event blocking mechanism to Document
• Make network loads initiated during parsing block document load (eg. script, link, img)
• Make subframe loads initiated during parsing block document load (ie. the enclosing document should not be unblocked until the frame has fired its load event)
• When parsing is complete, check whether the document load is blocked and fire a load event otherwise. If blocked, then only dispatch the load event when no more blockers remain.

[larsbergstrom]

@jdm is there a set of web platform tests that test these features? :-)

[jdm]

Unfortunately the ones that might be applicable seem to involve other unfinished features such as Element.style and inline event handlers (body.onload = ...). We'll probably want to write some simple ones ourselves.

[kmcallister]

This is heavily tied to #1879 (document lifecycle, incl. new HTML parser)

[jdm]

To some degree - I think we can create a separate DocumentLoader object that acts as an interface to the resource task and can be used by the parser and DOM objects.

[nox]

I did this.