style: Unprefix :-moz-selection.

Our implementation is totally not what the spec says, but totally what other
UAs do, see https://github.com/w3c/csswg-drafts/issues/2474.

So given this is causing webcompat pain, I think we should be pragmatic and just
unprefix this.

We could keep serialization and getComputedStyle with ::selection working with a
bit more effort, like we do for :-moz-placeholder, but I'd prefer not doing at
least the serialization bit, and just alias in nsCSSPseudoElements
:-moz-selection to selection too.

Bug: 509958
Reviewed-by: dbaron,xidorn
MozReview-Commit-ID: 6lxctozRDqv

components/style/gecko/pseudo_element_definition.mako.rs

@@ -216,11 +216,7 @@ impl PseudoElement {
         None
     }
 
-    /// Constructs an atom from a string of text, and whether we're in a
+    /// Constructs a pseudo-element from a string of text.
-    /// user-agent stylesheet.
-    ///
-    /// If we're not in a user-agent stylesheet, we will never parse anonymous
-    /// box pseudo-elements.
     ///
     /// Returns `None` if the pseudo-element is not recognised.
     #[inline]
@@ -234,6 +230,10 @@ impl PseudoElement {
                 return Some(${pseudo_element_variant(pseudo)})
             }
             % endfor
+            // Alias "-moz-selection" to "selection" at parse time.
+            "-moz-selection" => {
+                return Some(PseudoElement::Selection);
+            }
             _ => {
                 // FIXME: -moz-tree check should probably be
                 // ascii-case-insensitive.