Remove CSS Shapes runtime flag

Remove CSS Shapes runtime flag, since it is stable and was intended to ship a long time ago:

https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/N6_zzKAjLW4

Review URL: https://codereview.chromium.org/337093006

git-svn-id: svn://svn.chromium.org/blink/trunk@177773 bbb929c8-8fbe-4397-9dbb-9b2b20218538