Ensure original parser context is used when parsing resolved var() references
When resolving var() references at style building time we were not
using the CSSParserContext specific to where the var() came from.
This resulted in relative url()s not being able to resolve to an appropriate
absolute URL based on the source base URL.
This change attaches CSSParserContexts to var() references so they can be
used at style resolution time and appropriately resolve relative URLs.