CSS Variables: Avoid hasVariableReference call in applyProperty when disabled

There is no reason for checking variable references when variables have
been disabled. This patch shortcuts the check out of a hot code path to avoid
an unnecessary performance regression.

Review URL: https://chromiumcodereview.appspot.com/24225004

git-svn-id: svn://svn.chromium.org/blink/trunk@158068 bbb929c8-8fbe-4397-9dbb-9b2b20218538