style: Make container queries check content-box size

They were checking the border-box size (with paddings and borders), but
https://drafts.csswg.org/css-contain-3/#size-container says it should
be the content-box size.

Differential Revision: https://phabricator.services.mozilla.com/D163784

components/style/dom.rs

@@ -944,7 +944,7 @@ pub trait TElement:
         -> &<SelectorImpl as selectors::parser::SelectorImpl>::BorrowedNamespaceUrl;
 
     /// Returns the size of the primary box of the element.
-    fn primary_box_size(&self) -> euclid::default::Size2D<app_units::Au>;
+    fn primary_content_box_size(&self) -> euclid::default::Size2D<app_units::Au>;
 }
 
 /// TNode and TElement aren't Send because we want to be careful and explicit

components/style/gecko/wrapper.rs

@@ -1041,7 +1041,7 @@ impl<'le> TElement for GeckoElement<'le> {
     }
 
     #[inline]
-    fn primary_box_size(&self) -> Size2D<Au> {
+    fn primary_content_box_size(&self) -> Size2D<Au> {
         if !self.as_node().is_connected() {
             return Size2D::zero();
         }
@@ -1058,7 +1058,10 @@ impl<'le> TElement for GeckoElement<'le> {
             if frame.is_null() {
                 return Size2D::zero();
             }
-            Size2D::new(Au((**frame).mRect.width), Au((**frame).mRect.height))
+            let mut width = 0;
+            let mut height = 0;
+            bindings::Gecko_ContentSize(*frame, &mut width, &mut height);
+            Size2D::new(Au(width), Au(height))
         }
     }
 

components/style/stylesheets/container_rule.rs

@@ -201,7 +201,7 @@ impl ContainerCondition {
             }
         }
 
-        let size = potential_container.primary_box_size();
+        let size = potential_container.primary_content_box_size();
         let style = style.clone();
         TraversalResult::Done(ContainerLookupResult {
             element: potential_container,
@@ -464,7 +464,7 @@ impl<'a> ContainerSizeQuery<'a> {
         let box_style = style.get_box();
 
         let container_type = box_style.clone_container_type();
-        let size = e.primary_box_size();
+        let size = e.primary_content_box_size();
         match container_type {
             ContainerType::Size=> {
                 TraversalResult::Done(