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 188 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"
       }
     }
+  },
+  "options_tweaks_safariContentBlocker": {
+    "description": "Switch between classic and Safari's Content Blocking",
+    "message": "Use Safari's native <strong>Content Blocking</strong> (experimental)"
   }
 }