Implement Document.readyState. Prevent iframes from notifying the compositor after the initial parse. Fixes #1720. Fixes #3738.
auto merge of #3759 : jdm/servo/readystate, r=Ms2ger

...itor after the initial parse. Fixes #1720. Fixes #3738.

r? @Ms2ger or @Manishearth
Cleanup some Document methods.
Introduce LayoutDocumentHelpers::is_html_document_for_layout for LayoutElementHelpers::html_element_in_html_document_for_layout.
Move the methods on bare Document onto DocumentHelpers.
Cleanup Document::Title.