stylo: Include content rules from HTMLTableCellElement::WalkContentStyleRules

This also removes the TABLE_ATTRS_DIRTY optimization. Constructing nsMappedAttributes isn't really expensive and we do it all the time anyway.

components/style/gecko_bindings/bindings.rs

@@ -596,6 +596,10 @@ extern "C" {
                                                              RawGeckoElementBorrowed)
      -> RawServoDeclarationBlockStrongBorrowedOrNull;
 }
+extern "C" {
+    pub fn Gecko_GetExtraContentStyleDeclarations(element: RawGeckoElementBorrowed)
+     -> RawServoDeclarationBlockStrongBorrowedOrNull;
+}
 extern "C" {
     pub fn Gecko_GetAnimationRule(aElement: RawGeckoElementBorrowed,
                                   aPseudoTag: *mut nsIAtom,