Fix crash when calling cssText on styles with CSS variables

Extend RuntimeCSSEnabled's bitvector to accommodate
CSSPropertyVariable.

BUG=265419

Review URL: https://chromiumcodereview.appspot.com/21058002

git-svn-id: svn://svn.chromium.org/blink/trunk@155424 bbb929c8-8fbe-4397-9dbb-9b2b20218538