Provide a mechanism to indicate when geolocation / media access is in use

Since bug 1428754 landed, applications have no way to know whether geolocation and / or media devices are being accessed or blocked once the initial permission has been set. This will only get worse when we have peristent (ie, between sessions) storage of content settings.

We need to expand WebView.blockedContent and add the inverse (WebView.allowedContent ?), which is what this property was added for. This can be used by the application to indicate eg, whether a site is using the microphone or whether a site has been denied access to geolocation.