style: Use CascadeFlags for what they're for.

Now that we have an Element around on cascade, we can stop using the cascade
flags mechanism to pass various element-related state, like "is this element the
root", or "should it use the item-based display fixup".

That fixes handwaviness in the handling of those flags from style reparenting,
and code duplication to handle tricky stuff like :visited.

There are a number of other changes that are worth noticing:

 * skip_root_and_item_based_display_fixup is renamed to skip_item_display_fixup:

   TElement::is_root() already implies being the document element, which by
   definition is not native anonymous and not a pseudo-element.

   Thus, you never get fixed-up if your NAC or a pseudo, which is what the code
   tried to avoid, so the only fixup with a point is the item one, which is
   necessary.

 * The pseudo-element probing code was refactored to return early a
   Option::<CascadeInputs>::None, which is nicer than what it was doing.

 * The visited_links_enabled check has moved to selector-matching time. The rest
   of the checks aren't based on whether the element is a link, or are properly
   guarded by parent_style.visited_style().is_some() or visited_rules.is_some().

   Thus you can transitively infer that no element will end up with a :visited
   style, not even from style reparenting.

Anyway, the underlying reason why I want the element in StyleAdjuster is because
we're going to implement an adjustment in there depending on the tag of the
element (converting display: contents to display: none depending on the tag), so
computing that information eagerly, including a hash lookup, wouldn't be nice.

components/style/gecko/pseudo_element.rs

@@ -10,7 +10,7 @@
 
 use cssparser::{ToCss, serialize_identifier};
 use gecko_bindings::structs::{self, CSSPseudoElementType};
-use properties::{ComputedValues, PropertyFlags};
+use properties::{CascadeFlags, ComputedValues, PropertyFlags};
 use properties::longhands::display::computed_value::T as Display;
 use selector_parser::{NonTSPseudoClass, PseudoElementCascadeType, SelectorImpl};
 use std::fmt;
@@ -51,6 +51,15 @@ impl PseudoElement {
         PseudoElementCascadeType::Lazy
     }
 
+    /// The CascadeFlags needed to cascade this pseudo-element.
+    ///
+    /// This is only needed to support the broken INHERIT_ALL pseudo mode for
+    /// Servo.
+    #[inline]
+    pub fn cascade_flags(&self) -> CascadeFlags {
+        CascadeFlags::empty()
+    }
+
     /// Whether the pseudo-element should inherit from the default computed
     /// values instead of from the parent element.
     ///
@@ -128,7 +137,7 @@ impl PseudoElement {
     /// Whether this pseudo-element skips flex/grid container display-based
     /// fixup.
     #[inline]
-    pub fn skip_item_based_display_fixup(&self) -> bool {
+    pub fn skip_item_display_fixup(&self) -> bool {
         (self.flags() & structs::CSS_PSEUDO_ELEMENT_IS_FLEX_OR_GRID_ITEM) == 0
     }