Disable setting incognito prefs for extensions for now.

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 5364 matching lines @@
             "parameters": [
               {
                 "name": "details",
                 "type": "object",
                 "description": "What setting to change.",
                 "properties": {
                   "value": {
                     "description": "The value of the preference. <br>Note that every preference has a specific value type, which is described together with the preference. An extension should <em>not</em> set a preference value of a different type.",
                     "type": "any"
                   },
-                  "incognito": {
-                    "type": "boolean",
+                  "scope": {
+                    "type": "string",
+                    "enum": ["regular"],
                     "optional": true,
-                    "description": "Whether to modify the setting for the incognito session only (default false)."
+                    "description": "Where to set the preference (default: regular). 'regular' = preference for regular profile (which is inherited by the incognito profile if not overridden elsewhere)."
                   }
                 }
               },
               {
                 "name": "callback",
                 "type": "function",
                 "description": "Called after the preference has been set.",
                 "optional": true,
                 "parameters": []
               }
             ]
           },
           {
             "name": "clear",
             "type": "function",
             "description": "Clears the setting in the user preferences. This way default settings can become effective again.",
             "parameters": [
               {
                 "name": "details",
                 "type": "object",
-                "description": "What setting to clear.",
+                "description": "What preference to clear.",
                 "properties": {
-                  "incognito": {
-                    "type": "boolean",
+                  "scope": {
+                    "type": "string",
+                    "enum": ["regular"],
                     "optional": true,
-                    "description": "Whether to clear the setting for the incognito session only (default false)."
+                    "description": "Where to clear the preference (default: regular). 'regular' = preference for regular profile (which is inherited by the incognito profile if not overridden elsewhere)."
                   }
                 }
               },
               {
                 "name": "callback",
                 "type": "function",
                 "description": "Called after the preference has been cleared.",
                 "optional": true,
                 "parameters": []
               }
@@ skipping 168 matching lines @@
           {
             "type": "integer",
             "name": "tabId",
             "description": "The id of the tab that was detached."
           }
         ]
       }
     ]
   }
 ]