- Do not resolve raw headers upon `loadingFinished`, since they may still come later in
`responseReceivedExtraInfo`.
- Introduce separate promises for `encodedBodySize`, `transferSize` and `responseHeadersSize`.
- Make sure we resolve each of them either with data available
from the browser, or a fallback calculation.
- Set raw response headers for redirects on WebKit.
- Do not stall on cached responses in Chromium, they have erroneously set `hasExtraInfo` flag.
- Use `transferSize` that is available in Firefox protocol.
Currently, `loadstate` and `load` are two separate events in the protocol,
and are fired in this order. As a result, `waitForLoadState()` sometimes
resolves before the `'load'` event is fired, which is unexpected.
Also fixes a flaky test that assumed `load` event comes after `domcontentloaded`
for the empty page, which is not always a case in Chromium.
