Issue 3687 - Add string for content blocking option

_locales/en_US/messages.json

 {
   "add": {
     "description": "This is the label for the 'Add' buttons.",
     "message": "Add"
   },
   "add_domain_button": {
     "message": "Add domain"
   },
   "add_filter_button": {
     "message": "Add filter"
@@ skipping 92 matching lines @@
   },
   "notification_with_buttons": {
     "message": "Click 'OK' to open all links in this notification."
   },
   "notification_without_buttons": {
     "message": "Click on the notification to open all links in it."
   },
   "options": {
     "message": "Adblock Plus Options"
   },
+  "options_safariContentBlocker": {

[Sebastian Noack, 2016/03/09 17:04:31]

Nit: With the semantics in the file here, we don't use any prefix to indicate
the part of the app. In fact the other two strings starting with "options"
literally refer to buttons/links bringing up the options. 

+    "description": "Switch between classic and Safari's Content Blocking",
+    "message": "Use Safari's native Content Blocking (experimental)"
+  },
   "options_short": {
     "description": "Link to the options page from the popup balloon.",
     "message": "Options"
   },
   "out_of_date_lists": {
     "message": "Out-of-date lists are updated periodically."
   },
   "remove_button": {
     "description": "This is the label for the 'Remove selected' buttons.",
     "message": "Remove selected"
@@ skipping 78 matching lines @@
   "line": {
     "message": "Line $number$",
     "description": "Prepended to error messages to indicate the line number (e.g. \"Line 42: Some error on this line\")",
     "placeholders": {
       "number": {
         "content": "$1"
       }
     }
   }
 }