Introduce a separate preference for 'proxy server mode'

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 3618 matching lines @@
         "properties": {
           "url": {"type": "string", "optional": true, "description": "URL of he PAC file to be used."}
         }
       },
       {
         "id": "ProxyConfig",
         "type": "object",
         "description": "An object encapsulating a complete proxy configuration.",
         "properties": {
           "rules": {"$ref": "ProxyRules", "optional": true, "description": "The proxy rules describing this configuration."},
-          "autoDetect": {"type": "boolean", "optional": true, "description": "True if the proxy settings sbould be detected automatically."},
-          "pacScript": {"$ref": "PacScript", "optional": true, "description": "The proxy auto-config (PAC) script for this configuration."}
+          "pacScript": {"$ref": "PacScript", "optional": true, "description": "The proxy auto-config (PAC) script for this configuration."},
+          "mode": {
+            "type": "string",
+            "enum": ["direct", "auto_detect", "pac_script", "fixed_servers", "system"],
+            "description": "'direct' = Never use a proxy<br>'auto_detect' = Auto detect proxy settings<br>'pac_script' = Use specified PAC script<br>'fixed_servers' = Manually specify proxy servers<br>'system' = Use system proxy settings"
+          }
         }
       }
     ],
     "functions": [
       {
         "name": "useCustomProxySettings",
         "type": "function",
         "description": "Apply the given proxy configuration.",
         "parameters": [
           {"name": "config", "$ref": "ProxyConfig"}
@@ skipping 714 matching lines @@
                   "token": { "type": "string", "optional": true }
                 }
               }
             ]
           }
         ]
       }
     ]
   }
 ]