Clean up after getPropertyCSSValue

This patch removes the idl interfaces for getPropertyCSSValue and friends
(i.e. those behind the runtime flag CSSValue), and removes code that only
existed to support this.

BUG=331608

Review URL: https://codereview.chromium.org/717933002

git-svn-id: svn://svn.chromium.org/blink/trunk@185192 bbb929c8-8fbe-4397-9dbb-9b2b20218538