Rollup merge of #17010 - CJKu:bug-1348490, r=emilio

Stylo: match both ::placehoder & ::moz-placeholder for placeholder ps…

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The first patch of "Bug 1348490 - stylo: need support for ::-moz-placeholder pseudo element"

Part 2 need to be landed immediately after Part 1 be merged into servo repo:
https://reviewboard.mozilla.org/r/141264/diff/9#index_header

Bugzilla link:
https://bugzilla.mozilla.org/show_bug.cgi?id=1348490
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- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] These changes do not require tests because _____

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components/style/stylist.rs

@@ -473,7 +473,7 @@ impl Stylist {
     {
         let map = if let Some(pseudo) = selector.pseudo_element() {
             self.pseudos_map
-                .entry(pseudo.clone())
+                .entry(pseudo.canonical())
                 .or_insert_with(PerPseudoElementSelectorMap::new)
                 .borrow_for_origin(&stylesheet.origin)
         } else {
@@ -665,8 +665,9 @@ impl Stylist {
                                 -> Option<StrongRuleNode>
         where E: TElement
     {
+        let pseudo = pseudo.canonical();
         debug_assert!(pseudo.is_lazy());
-        if self.pseudos_map.get(pseudo).is_none() {
+        if self.pseudos_map.get(&pseudo).is_none() {
             return None
         }
 
@@ -698,7 +699,7 @@ impl Stylist {
         let mut matching_context =
             MatchingContext::new(MatchingMode::ForStatelessPseudoElement, None);
         self.push_applicable_declarations(element,
-                                          Some(pseudo),
+                                          Some(&pseudo),
                                           None,
                                           None,
                                           AnimationRules(None, None),