Move contentSettings extension API out of experimental

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "unprivileged": true,
     "types": [
       {
         "id": "MessageSender",
         "type": "object",
         "description": "An object containing information about the script context that sent a message or request.",
         "properties": {
@@ skipping 7889 matching lines @@
                   }
                 }
               }
             ]
           }
         ]
       }
     ]
   },
   {
-    "namespace": "experimental.contentSettings",
+    "namespace": "contentSettings",
     "types": [
       {
         "id": "ResourceIdentifier",
         "type": "object",
         "properties": {
           "id": {
             "type": "string",
             "description": "The resource identifier for the given content type."
           },
           "description": {
@@ skipping 781 matching lines @@
                 "optional": "true",
                 "description": "The MHTML data as a Blob."
               }
             ]
           }
         ]
       }
     ]
   }
 ]