Suppress reflows before RefreshTick or FirstLoad

This fixes a bug where partially loaded content is displayed to the user
before it should be, usually before stylesheets have loaded. This commit
supresses reflows until either FirstLoad or RefreshTick, whichever comes
first.

Unfortunately, hit_test and mouse_over did not do reflows if they were
necessary, and so by suppressing the initial spurious reflows, these
methods started to panic without a display list to query. This patch
also transforms these into queries similar to the other existing
queries.

components/script/dom/xmldocument.rs

@@ -6,7 +6,6 @@ use document_loader::DocumentLoader;
 use dom::bindings::codegen::Bindings::DocumentBinding::DocumentMethods;
 use dom::bindings::codegen::Bindings::WindowBinding::WindowMethods;
 use dom::bindings::codegen::Bindings::XMLDocumentBinding::{self, XMLDocumentMethods};
-use dom::bindings::error::Fallible;
 use dom::bindings::global::GlobalRef;
 use dom::bindings::inheritance::Castable;
 use dom::bindings::js::{Root, RootedReference};