Add request_id to HttpRequestInfo and pass it to the NetworkDelegate for events.

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 3550 matching lines @@
               "url": {"type": "string"},
               "method": {"type": "string", "description": "Standard HTTP method."},
               "tabId": {"type": "integer", "description": "The ID of the tab in which the request takes place. Set to null if the request isn't related to a tab."},
               "type": {"type": "string", "enum": ["main_frame", "sub_frame", "stylesheet", "script", "image", "object", "other"], "description": "How the requested resource will be used."},
               "timeStamp": {"type": "number", "description": "The time when the browser was about to make the request, in milliseconds since the epoch."}
             }
           }
         ]
       },
       {
+        "name": "onBeforeSendHeaders",
+        "type": "function",
+        "description": "Fires before sending an HTTP request, once the request headers are available.",
+        "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"},
+              "timeStamp": {"type": "number", "description": "The time when the browser was about to make the request, in milliseconds since the epoch."}
+            }
+          }
+        ]
+      },
+      {
         "name": "onRequestSent",
         "type": "function",
         "description": "Fires when a request is sent to the server.",
         "parameters": [
           {
             "type": "object",
             "name": "details",
             "properties": {
               "requestId": {"type": "string", "description": "The ID of the request."},
               "url": {"type": "string"},
@@ skipping 1247 matching lines @@
           {
             "type": "integer",
             "name": "tabId",
             "description": "The id of the tab that was detached."
           }
         ]
       }
     ]
   }
 ]