Implement chrome.systemPrivate.getIncognitoModeAvailability extension API function

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "types": [
       {
         "id": "MessageSender",
         "type": "object",
         "description": "An object containing information about the script context that sent a message or request.",
         "properties": {
           "tab": {"$ref": "Tab", "optional": true, "description":"This property will <b>only</b> be present when the connection was opened from a tab or content script."},
@@ skipping 1165 matching lines @@
             "enum": ["normal", "minimized", "maximized"]
             }
         }
       }
     ],
     "properties": {
       "WINDOW_ID_NONE": {
         "type": "integer",
         "value": "-1",
         "description": "The windowId value that represents the absence of a chrome browser window."
+      },
+      "incognitoModeAvailability": {
+        "$ref": "ChromeSetting",
+        "value": ["incognitoModeAvailability", {"type": "string", "enum": ["enabled", "disabled", "forced"]}],
+        "description": "Read-only property that exposes whether the incognito mode is available to windows. One of 'enabled', 'disabled' (user cannot browse pages in Incognito mode), 'forced' (all pages/sessions are forced into Incognito mode)."
       }
     },
     "functions": [
       {
         "name": "get",
         "type": "function",
         "description": "Gets details about a window.",
         "parameters": [
           {"type": "integer", "name": "windowId", "minimum": 0},
           {
@@ skipping 8039 matching lines @@
                   }
                 }
               }
             ]
           }
         ]
       }
     ]
   }
 ]