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": {"$ref": "Tab", "optional": true, "description":"This property will <b>only</b> be present when the connection was opened from a tab or content script."},
@@ skipping 9131 matching lines @@
                       "description": "Page url"
                     }
                   }
                 }
               }
             ]
           }
         ]
       }
     ]
+  },
+  {
+    "namespace": "systemPrivate",
+    "nodoc": true,
+    "types": [],
+    "functions": [],
+    "properties": {
+      "incognitoModeAvailability": {
+        "$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)."
+      }
+    }
   }
 ]