webRequest.onAuthRequired listeners can provide authentication credentials.

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 5804 matching lines @@
           },
           "requestHeaders": {
             "$ref": "HttpHeaders",
             "optional": true,
             "description": "Only used as a response to the onBeforeSendHeaders event. If set, the request is made with these request headers instead."
           },
           "responseHeaders": {
             "$ref": "HttpHeaders",
             "optional": true,
             "description": "Only used as a response to the onHeadersReceived event. If set, the server is assumed to have responsed with these response headers instead. Only return <code>responseHeaders</code> if you really want to modify the headers in order to limit the number of conflicts (only one extension may modify <code>responseHeaders</code> for each request)."
+          },
+          "authCredentials": {
+            "type": "object",
+            "description": "Only used as a response to the onAuthRequired event. If set, the request is made using the supplied credentials.",
+            "optional": true,
+            "properties": {
+              "username": {"type": "string"},
+              "password": {"type": "string"}
+            }
           }
         }
       }
     ],
     "functions": [
       {
         "name": "addEventListener",
         "nodoc": true,
         "type": "function",
         "description": "Used internally to implement the special form of addListener for the webRequest events.",
@@ skipping 248 matching lines @@
             "name": "filter",
             "description": "A set of filters that restricts the events that will be sent to this listener."
           },
           {
             "type": "array",
             "optional": true,
             "name": "extraInfoSpec",
             "description": "Array of extra information that should be passed to the listener function.",
             "items": {
               "type": "string",
-              "enum": ["responseHeaders"]
+              "enum": ["responseHeaders", "blocking"]
             }
           }
-        ]
+        ],
+        "returns": {
+          "$ref": "BlockingResponse",
+          "description": "If \"blocking\" is specified in the \"extraInfoSpec\" parameter, the event listener should return an object of this type.",
+          "optional": true
+        }
       },
       {
         "name": "onResponseStarted",
         "type": "function",
         "description": "Fired when the first byte of the response body is received. For HTTP requests, this means that the status line and response headers are available.",
         "parameters": [
           {
             "type": "object",
             "name": "details",
             "properties": {
@@ skipping 2841 matching lines @@
                 "optional": "true",
                 "description": "The MHTML data as a Blob."
               }
             ]
           }
         ]
       }
     ]
   }
 ]