Specify runtime enabled CSS properties in CSSProperties.in

This change makes CSSProperties.in aware of runtime enabled properties,
making configuring these slightly easier and more discoverable. I've
moved the logic to CSSPropertyMetadata instead of adding another code
generator script just for these since CSSPropertyMetadata is a good fit
for isEnabledProperty anyway.

BUG=396992

Review URL: https://codereview.chromium.org/512603002

git-svn-id: svn://svn.chromium.org/blink/trunk@180980 bbb929c8-8fbe-4397-9dbb-9b2b20218538