Change the first parameter of savePage.saveAsMHTML to be a dictionary.

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 9044 matching lines @@
         ]
       }
     ]
   },
   {
     "namespace": "experimental.savePage",
     "functions": [
       {
         "name": "saveAsMHTML",
         "type": "function",
-        "description": "Saves the content of the tab with given id to MHTML.",
+        "description": "Saves the content of the tab with given id as MHTML.",
         "parameters": [
           {
-            "name": "tabId",
-            "type": "integer",
-            "minimum": 0,
-            "description": "The id of the tab to save."
+            "name": "details",
+            "type": "object",
+            "properties": {
+              "tabId": {
+                "type": "integer",
+                "minimum": 0,
+                "description": "The id of the tab to save as MHTML."
+              }
+            }
           },
           {
             "name": "callback",
             "type": "function",
             "description": "Called when the MHTML has been generated.",
             "parameters": [
               {
                 "name": "mhtmlData",
                 "type": "blob",
                 "optional": "true",
@@ skipping 148 matching lines @@
                   }
                 }
               }
             ]
           }
         ]
       }
     ]
   }
 ]