Ensure ref tests wait to write their PNG until the page has fully loaded and reflowed

Add a LoadComplete message so that script informs the constellation, which can then inform the compositor (and anyone else, later) about the completion of loading a page. This is important for ref tests, which should not emit a PNG until load has completed, even if we perform a composite before then.

Note that we must *also* wait for the `ready_state` to indicate that additionally layout has completed any associated reflow with the page, otherwise we could get the `LoadComplete` while the final reflow is still in progress and thus would write out the `n-1`'th reflow result.

r? @pcwalton