An animator class declares all the input properties for all its proxies which may include css properties. The animator instance is able to read these values in worklet scope so we need a way to provide a read-only copies of these values in the worklet scope.

Here is an outline of how this may be achieved:
 - Expose StylePropertyMap.idl to Animation Worklet. This will be the type used for proxy.styleMap (to be defined in CompositorProxy.idl)
 - Add a new implementation of ImmutableStylePropertyMap 
 with an internal  HashMap<CSSPropertyID, CSSStyleValueVector> 
 - This backing map is initialized during object construction from underlying node's *computed style* values of properties that we are interested in. This is similar to |FilteredComputedStylePropertyMap| but instead of keeping a reference to underlying node, we copy values at initialization.
 - This object can be safely handed over to "Animation Worklet Thread" now.
 - Every time we need to update these value for the other worklet (e.g., every commit cycle) we make a new copy with fresh values and send over. Probably only need to do this if the values have changed.