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 103 matching lines @@
     "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     "message": "Out-of-date lists are updated periodically."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"
   },
+  "safari_content_blocker": {
+    "description": "Switch between classic and Safari's Content Blocking",
+    "message": "Use Safari's native Content Blocking (experimental)"
+  },
   "show_block_element_menu": {
     "message": "Show 'Block Element' right-click menu item"
   },
   "stats_label_page": {
     "description": "This shows the number of blocked requests on a specific page in the popup.",
     "message": "<strong>$number$</strong> on this page",
     "placeholders": {
       "number": {
         "content": "$1"
       }
@@ skipping 67 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"
       }
     }
   }
 }