Enable (optional) blocking of webrequests in case a PAC script cannot be fetched or is invalid.

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 3982 matching lines @@
           "fallbackProxy": {"$ref": "ProxyServer", "optional": true, "description": "The proxy server to be used for everthing else or if any of the specific proxyFor... is not specified."},
           "bypassList": {"type": "array", "items": {"type": "string"}, "optional": true, "description": "List of servers to connect to without a proxy server."}
         }
       },
       {
         "id": "PacScript",
         "type": "object",
         "description": "An object holding proxy auto-config information. Exactly one of the fields should be non-empty.",
         "properties": {
           "url": {"type": "string", "optional": true, "description": "URL of the PAC file to be used."},
-          "data": {"type": "string", "optional": true, "description": "A PAC script."}
+          "data": {"type": "string", "optional": true, "description": "A PAC script."},
+          "mandatory": {"type": "boolean", "optional": true, "description": "If true, an invalid PAC script will prevent the network stack from falling back to direct connections. Defaults to false."}
         }
       },
       {
         "id": "ProxyConfig",
         "type": "object",
         "description": "An object encapsulating a complete proxy configuration.",
         "properties": {
           "rules": {"$ref": "ProxyRules", "optional": true, "description": "The proxy rules describing this configuration. Use this for 'fixed_servers' mode."},
           "pacScript": {"$ref": "PacScript", "optional": true, "description": "The proxy auto-config (PAC) script for this configuration. Use this for 'pac_script' mode."},
           "mode": {
@@ skipping 1344 matching lines @@
           {
             "type": "integer",
             "name": "tabId",
             "description": "The id of the tab that was detached."
           }
         ]
       }
     ]
   }
 ]