1413730 - Add leak testing that logs URLs of leaked documents

Status: NEW

(Testing :: General, enhancement, P3)

Description

[Andrew McCreight [:mccr8]]

Any big leak like bug 1398563 that involves windows and documents ends up producing a completely useless list of classes.

This is bad for a number of reasons:
- If a class near the start of the alphabet gets added or removed or renamed, then a new bug will get filed.
- It seems like APZ is to blame (or whatever) because there's an APZ class near the start of the alphabet. 
- Completely unrelated leaks get bucketed together. bug 1398563 is two different leaks. I've seen a single bugzilla bug shift from one leak to another over a series of months.
- You can't tell what test is leaking, if there is a particular one.

I think if we produced some kind of error that gave the URL for any documents that leak then that could be used to produce the error, even overriding the existing leakcheck message. This would work like the "Leaked URLs" list we have right now, except that seems to include a bunch of chrome junk (chrome://global/content/bindings/scrollbar.xml, chrome://global/skin/scrollbars.css, etc.) that is not informative.