DevTools: [CSS] do not include runtime-flag guarded properties in a supported list

As of today, we get all the properties from CSSProperties.in to generate
supported css properties. However, some of them are guarded by the runtime
flag. This patch avoids adding these properties to the front-end.

BUG=419326
R=apavlov

Review URL: https://codereview.chromium.org/619613008

git-svn-id: svn://svn.chromium.org/blink/trunk@183068 bbb929c8-8fbe-4397-9dbb-9b2b20218538