Bug 1322945 - Remove the requirement that the parent styles must be current to style a subtree. r=heycam

Sometimes Gecko eagerly styles things without processing pending restyles first. In
general we'd like to avoid this, but there can be good reasons (for example, needing
to construct a frame for some small piece of newly-added content in order to do something
specific with that frame, but not wanting to flush all of layout). Just handle it.

MozReview-Commit-ID: EjXs0M4855Q

components/style/matching.rs

@@ -733,7 +733,6 @@ pub trait MatchMethods : TElement {
         // Get our parent's style.
         let parent_data = parent.as_ref().map(|x| x.borrow_data().unwrap());
         let parent_style = parent_data.as_ref().map(|d| {
-            debug_assert!(d.has_current_styles());
             &d.styles().primary.values
         });