style: Micro-optimize is_root to avoid poking at the parent in the co…

…mmon case.

We can discard the common case of an element having another element or document
fragment parent, which is the common case.

Then we can discard detached subtrees looking at is_in_document().

The only case where we have a non-content parent is the document case:

  https://searchfox.org/mozilla-central/rev/033d45ca70ff32acf04286244644d19308c359d5/dom/base/Element.cpp#1683

Differential Revision: https://phabricator.services.mozilla.com/D2505

components/style/gecko/wrapper.rs

@@ -2074,15 +2074,15 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
 
     #[inline]
     fn is_root(&self) -> bool {
-        let parent_node = match self.as_node().parent_node() {
-            Some(parent_node) => parent_node,
-            None => return false,
-        };
+        if self.as_node().get_bool_flag(nsINode_BooleanFlag::ParentIsContent) {
+            return false;
+        }
 
-        if !parent_node.is_document() {
+        if !self.as_node().is_in_document() {
             return false;
         }
 
+        debug_assert!(self.as_node().parent_node().map_or(false, |p| p.is_document()));
         unsafe { bindings::Gecko_IsRootElement(self.0) }
     }