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/dom.rs

@@ -163,10 +163,10 @@ pub trait TElement : PartialEq + Debug + Sized + Copy + Clone + ElementExt + Pre
     /// traversal. Returns the number of children left to process.
     fn did_process_child(&self) -> isize;
 
-    /// Returns true if this element's style is display:none.
+    /// Returns true if this element's style is display:none. Panics if
+    /// the element has no style.
     fn is_display_none(&self) -> bool {
         let data = self.borrow_data().unwrap();
-        debug_assert!(data.has_current_styles());
         data.styles().is_display_none()
     }
 

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
         });