style: Don't apply property restrictions to pseudo-elements in UA stylesheets.

And remove some of the ::placeholder and ::cue hacks where we need to use
!important to make the property not apply for content but apply on UA sheets.

The comment about the white-space property was wrong, we don't enforce it with
!important in the UA stylesheets for <input> (we do for <textarea> though), so
I've kept the flag since it really applies.

Differential Revision: https://phabricator.services.mozilla.com/D37717

components/style/properties/cascade.rs

@@ -153,7 +153,7 @@ where
                         // longhands are only allowed if they have our
                         // restriction flag set.
                         if let PropertyDeclarationId::Longhand(id) = declaration.id() {
-                            if !id.flags().contains(restriction) {
+                            if !id.flags().contains(restriction) && cascade_level.origin() != Origin::UserAgent {
                                 return None;
                             }
                         }