webRequest.onAuthRequired listeners can provide authentication credentials.

chrome/common/extensions/api/extension_api.json

Index: chrome/common/extensions/api/extension_api.json
diff --git a/chrome/common/extensions/api/extension_api.json b/chrome/common/extensions/api/extension_api.json
index d8e729937627fdc0f9721979e550e9de4ceb8a95..b04fa1122e610d13dd6c6787c260988251e82a4c 100644
--- a/chrome/common/extensions/api/extension_api.json
+++ b/chrome/common/extensions/api/extension_api.json
@@ -5822,6 +5822,15 @@
             "$ref": "HttpHeaders",
             "optional": true,
             "description": "Only used as a response to the onHeadersReceived event. If set, the server is assumed to have responsed with these response headers instead. Only return <code>responseHeaders</code> if you really want to modify the headers in order to limit the number of conflicts (only one extension may modify <code>responseHeaders</code> for each request)."
+          },
+          "authCredentials": {
+            "type": "object",
+            "description": "Only used as a response to the onAuthRequired event. If set, the request is made using the supplied credentials.",
+            "optional": true,
+            "properties": {
+              "username": {"type": "string"},
+              "password": {"type": "string"}
+            }
           }
         }
       }
@@ -6090,10 +6099,15 @@
             "description": "Array of extra information that should be passed to the listener function.",
             "items": {
               "type": "string",
-              "enum": ["responseHeaders"]
+              "enum": ["responseHeaders", "blocking"]
             }
           }
-        ]
+        ],
+        "returns": {
+          "$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",