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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- [ ] These changes fix #__ (github issue number if applicable).

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components/style/gecko/wrapper.rs

@@ -1369,7 +1369,7 @@ 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,
+            Some(ref pseudo) => *pseudo == pseudo_element.canonical(),
             None => false,
         }
     }