fix(network): make allHeaders wait until all header are available
Before, calling `allHeaders()` from `page.on('request')` would yield
provisional headers instead.
With these changes:
- In Firefox, all headers are available immediately.
- In Chromium, all headers are available upon requestWillBeSentExtraInfo.
- In WebKit, all headers are available upon responseReceived.
- In all browsers, intercepted requests use "provisional" headers
as all headers, since there is no network stack to change the headers.
Drive-by: migrated Chromium to `hasExtraInfo` flags that simplifies
the logic quite a bit.
660516d22a