style: Set WillChangeBits::TRANSFORM for individual transform.

We always check StyleWillChangeBits_TRANSFORM bit together with a
transform-like property set, so using WillChangeBits::TRANSFORM bit to
represent all transform-like properties is ok.

However, it seems the new test case works well even if we don't have this
patch. I still add it for individual transform properties to make sure
the test coverage is enough anyway.

Differential Revision: https://phabricator.services.mozilla.com/D47509

components/style/values/specified/box.rs

@@ -1153,7 +1153,9 @@ bitflags! {
 fn change_bits_for_longhand(longhand: LonghandId) -> WillChangeBits {
     let mut flags = match longhand {
         LonghandId::Opacity => WillChangeBits::OPACITY,
-        LonghandId::Transform => WillChangeBits::TRANSFORM,
+        LonghandId::Transform | LonghandId::Translate | LonghandId::Rotate | LonghandId::Scale => {
+            WillChangeBits::TRANSFORM
+        }
         _ => WillChangeBits::empty(),
     };