Rename CSS_VARIABLE_NAME to CSS_VARIABLE_REFERENCE and related functions

This patch does not change existing behaviour.
The motivation for this rename is in preparation for adding a fallback
component to variable references. This means they won't be characterised
soley by the name of the variable they're referencing anymore and should
be named more appropriately.

BUG=330127

Review URL: https://codereview.chromium.org/132693026

git-svn-id: svn://svn.chromium.org/blink/trunk@165616 bbb929c8-8fbe-4397-9dbb-9b2b20218538