Auto merge of #18893 - emilio:bye-can-be-fragmented, r=SimonSapin

style: Remove TNode::set_can_be_fragmented and TNode::can_be_fragmented.

Replace them instead by a computed value flag, the same way as the
IS_IN_DISPLAY_NONE_SUBTREE flag works.

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components/style/properties/gecko.mako.rs

@@ -300,10 +300,6 @@ impl ComputedValuesInner {
         !self.get_box().gecko.mBinding.mRawPtr.is_null()
     }
 
-    // FIXME(bholley): Implement this properly.
-    #[inline]
-    pub fn is_multicol(&self) -> bool { false }
-
     pub fn to_declaration_block(&self, property: PropertyDeclarationId) -> PropertyDeclarationBlock {
         let value = match property {
             % for prop in data.longhands: