Issue 3687 - Add experimental support for Safari content blockers

_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     "message": "Out-of-date lists are updated periodically."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": {

[Sebastian Noack, 2016/05/12 11:12:22]

Why did you remove the message? Also what's about the message to restart Safari?

[kzar, 2016/05/17 15:15:36]

The strings are provided by adblockplusui/locale/*/options.json, no need to
duplicate them here.

[Sebastian Noack, 2016/05/17 18:35:24]

Those strings aren't imported yet. And even if we add the new options behind a
flag soon, though unlikely before the next release, the strings there won't be
translated as they aren't final yet.

[kzar, 2016/05/17 19:20:21]

The strings are imported, I added "adblockplusui/locale/*/options.json = =*" to
metadata.common in Patch Set 1. Would you rather I duplicate them here instead?

[Sebastian Noack, 2016/05/18 07:03:29]

I see. But still the strings there won't be translated. So yes, we have to
duplicate them here.

[kzar, 2016/05/18 08:02:48]

Done.

-    "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"
   },
   "show_devtools_panel": {
     "message": "Show 'Adblock Plus' panel in the developer tools"
   },
   "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": {
@@ skipping 70 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"
       }
     }
   }
 }