Mark Feature Policy spec as worth prototyping. (#69)

Fixes #24.

activities.json

@@ -69,8 +69,8 @@
     "ciuName": null,
     "description": "This specification defines a mechanism that allows developers to selectively enable and disable use of various browser features and APIs.",
     "mozBugUrl": "https://bugzilla.mozilla.org/show_bug.cgi?id=1390801",
-    "mozPosition": "under consideration",
-    "mozPositionDetail": null,
+    "mozPosition": "worth prototyping",
+    "mozPositionDetail": "This specification allows finer-grained control over certain features; some of these features require user consent, and some have privacy and/or security implications.  Allowing this control could allow pages that use these features (in many cases, following user consent) to prevent cross-origin subframes (which are often ads, and which often do tracking) from getting the same permission as the toplevel page.  Having the control also makes it easier for future specifications to be designed so that this behavior is the default, but can be overridden in the other direction to allow the subframe to use the feature.  The control provided by this specification also allows sites to exercise CSP-like control over their own content's use of these features.",
     "mozPositionIssue": 24,
     "org": "W3C",
     "title": "Feature Policy",