style: Remove the concept of 'canonical' pseudos.

We only have this so that ::-moz-placeholder keeps serializing as
::-moz-placeholder, but I don't think anybody really cares.

Edge aliases ::-webkit-input-placeholder to ::-ms-input-placeholder at parse
time as well, as can be seen in:

```
let s = document.createElement('style');
s.innerHTML = `input::-webkit-input-placeholder { color: red };`;
document.body.appendChild(s);
document.body.innerHTML = s.sheet.cssRules[0].cssText;
```

And I think this is more consistent with what we do for CSS properties that are
aliases.

Differential Revision: https://phabricator.services.mozilla.com/D2595

components/style/gecko/pseudo_element.rs

@@ -162,15 +162,6 @@ impl PseudoElement {
         self.is_anon_box() && !self.is_tree_pseudo_element()
     }
 
-    /// Covert non-canonical pseudo-element to canonical one, and keep a
-    /// canonical one as it is.
-    pub fn canonical(&self) -> PseudoElement {
-        match *self {
-            PseudoElement::MozPlaceholder => PseudoElement::Placeholder,
-            _ => self.clone(),
-        }
-    }
-
     /// Property flag that properties must have to apply to this pseudo-element.
     #[inline]
     pub fn property_restriction(&self) -> Option<PropertyFlags> {

components/style/gecko/pseudo_element_definition.mako.rs

@@ -234,6 +234,9 @@ impl PseudoElement {
             "-moz-selection" => {
                 return Some(PseudoElement::Selection);
             }
+            "-moz-placeholder" => {
+                return Some(PseudoElement::Placeholder);
+            }
             _ => {
                 // FIXME: -moz-tree check should probably be
                 // ascii-case-insensitive.

components/style/gecko/wrapper.rs

@@ -2267,7 +2267,7 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
         // match the proper pseudo-element, given how we rulehash the stuff
         // based on the pseudo.
         match self.implemented_pseudo_element() {
-            Some(ref pseudo) => *pseudo == pseudo_element.canonical(),
+            Some(ref pseudo) => *pseudo == *pseudo_element,
             None => false,
         }
     }

components/style/stylist.rs

@@ -898,7 +898,6 @@ impl Stylist {
     where
         E: TElement,
     {
-        let pseudo = pseudo.canonical();
         debug_assert!(pseudo.is_lazy());
 
         // Apply the selector flags. We should be in sequential mode
@@ -1892,7 +1891,7 @@ impl ElementAndPseudoRules {
         let map = match pseudo_element {
             None => &mut self.element_map,
             Some(pseudo) => self.pseudos_map
-                .get_or_insert_with(&pseudo.canonical(), || Box::new(SelectorMap::new())),
+                .get_or_insert_with(pseudo, || Box::new(SelectorMap::new())),
         };
 
         map.insert(rule, quirks_mode)
@@ -1906,7 +1905,7 @@ impl ElementAndPseudoRules {
     #[inline]
     fn rules(&self, pseudo: Option<&PseudoElement>) -> Option<&SelectorMap<Rule>> {
         match pseudo {
-            Some(pseudo) => self.pseudos_map.get(&pseudo.canonical()).map(|p| &**p),
+            Some(pseudo) => self.pseudos_map.get(pseudo).map(|p| &**p),
             None => Some(&self.element_map),
         }
     }
@@ -2190,7 +2189,7 @@ impl CascadeData {
                                 precomputed_pseudo_element_decls
                                     .as_mut()
                                     .expect("Expected precomputed declarations for the UA level")
-                                    .get_or_insert_with(&pseudo.canonical(), Vec::new)
+                                    .get_or_insert_with(pseudo, Vec::new)
                                     .push(ApplicableDeclarationBlock::new(
                                         StyleSource::from_rule(locked.clone()),
                                         self.rules_source_order,