Use the chrome.experimental.webRequest module to intercept, block, or modify requests in-flight. This module is still very much experimental. For information on how to use experimental APIs, see the chrome.experimental.* APIs page.

A note about timestamps

It's important to note that some technical oddities in the OS's handling of distinct Chrome processes can cause the clock to be skewed between the browser itself and extension processes. That means that WebRequest's events' timeStamp property is only guaranteed to be internally consistent. Comparing one event to another event will give you the correct offset between them, but comparing them to the current time inside the extension (via (new Date()).getTime(), for instance) might give unexpected results.