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/pseudo_element.rs

@@ -102,6 +102,15 @@ impl PseudoElement {
     pub fn is_precomputed(&self) -> bool {
         self.is_anon_box()
     }
+
+    /// 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(),
+        }
+    }
 }
 
 impl ToCss for PseudoElement {