Bug 1479398: Micro-optimize is_root to avoid poking at the parent in the common case. r=xidorn

Summary:
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

Reviewers: xidorn

Tags: #secure-revision

Bug #: 1479398

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

MozReview-Commit-ID: H2mWHBmk5g1

servo/components/style/gecko/wrapper.rs

@@ -2087,15 +2087,15 @@ 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 !parent_node.is_document() {
+        if self.as_node().get_bool_flag(nsINode_BooleanFlag::ParentIsContent) {
             return false;
         }
 
+        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) }
     }