This callback is invoked when the page requests a resource. The first argument to the callback is the requestData metadata object. The second argument is the networkRequest object itself.
The requestData metadata object contains these properties:
id : the number of the requested resource
method : http method
url : the URL of the requested resource
time : Date object containing the date of the request
headers : list of http headers
The networkRequest object contains these functions:
abort() : aborts the current network request. Aborting the current network request will invoke onResourceError callback.
changeUrl(newUrl) : changes the current URL of the network request. By calling networkRequest.changeUrl(newUrl), we can change the request url to the new url. This is an excellent and only way to provide alternative implementation of a remote resource. (see Example-2)
setHeader(key, value)
