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 212 matching lines @@
         ],
         "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": "onAuthRequired",
         "type": "function",
-        "description": "Fired when an authentication failure was received. Depending on whether the user provides credentials, the request is either reissued or cancelled.",
+        "description": "Fired when an authentication failure was received. The listener has three options: it can provide authentication credentials, it can cancel the request and display the error page, or it can take no action on the challenge. If bad user credentials are provided, this may be called multiple times for the same request.",
         "parameters": [
           {
             "type": "object",
             "name": "details",
             "properties": {
               "requestId": {"type": "string", "description": "The ID of the request. Request IDs are unique within a browser session. As a result, they could be used to relate different events of the same request."},
               "url": {"type": "string"},
               "method": {"type": "string", "description": "Standard HTTP method."},
               "frameId": {"type": "integer", "description": "0 indicates the request happens in the main frame; a positive value indicates the ID of a subframe in which the request happens. If the document of a (sub-)frame is loaded (<code>type</code> is <code>main_frame</code> or <code>sub_frame</code>), <code>frameId</code> indicates the ID of this frame, not the ID of the outer frame. Frame IDs are unique within a tab."},
               "tabId": {"type": "integer", "description": "The ID of the tab in which the request takes place. Set to -1 if the request isn't related to a tab."},
@@ skipping 15 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."
               }
             ]
           }
         ]
       }
     ]
   }
 ]