Implement Document.readyState. Prevent iframes from notifying the compos...

[jdm]

...itor after the initial parse. Fixes #1720. Fixes #3738.

r? @Ms2ger or @Manishearth

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/2938

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[larsbergstrom]

Can either @Ms2ger or @Manishearth please take a look at this? I believe it's blocking a PR from @kmcallister which is in turn blocking a PR from @mbrubeck :-)

[Manishearth]

In a way #3776 blocks this one, lengthening the chain ;P

[Manishearth]

@mbrubeck You probably know this, but be a bit careful about the interactive ready state (scroll up) for #3721 once this merges.

[hsivonen]

As noted in another PR, in the case of Gecko, I found that it would be beneficial to have state tracking across the navigation. If such a state tracking object existed, it would be logical for the true readyState to be kept on that object such that once Document is created, it would get a reference to the associated navigation tracking object and the Document's readyState could then call through to that object to ask for the state.