Hi Nikita,
You can use chrome.devtools.network.onNavigated:
http://code.google.com/chrome/extensions/trunk/devtools.network.html#event-onNavigated
It's fired at about the same time that onReset used to when the main
frame navigates to a new URL. Note that unlike onReset, this event is
additionally fired on the DevTools start up, when the front-end
retrieves resource data from the back-end, though you may miss it in
case your extension is loaded after the front-end is initialized.
Best regards,
Andrey.