Fix reading invalid union member in CSSParser::parseValue()

Check for variable token even when integer token is expected in
CSS parser.

BUG=232393

Review URL: https://chromiumcodereview.appspot.com/19976007

git-svn-id: svn://svn.chromium.org/blink/trunk@154871 bbb929c8-8fbe-4397-9dbb-9b2b20218538