Enable URLRequest::Delegate::OnConnected to allow ERR_IO_PENDING

This will allow the URLLoader to call an off-process function to
(possibly) populate the request with additional headers (specifically
client hints as part of the ACCEPT_CH H2/3 frame from Client Hints
Reliability).

Here's the new:

 * HttpNetworkTransaction now has a new state (STATE_AFTER_INIT_STREAM)
   that always comes after STATE_INIT_STREAM_COMPLETE.

 * There is now a callback
   (HttpNetworkTransaction::ResumeAfterConnected) that is passed into
   the connected_callback_ (up through to URLRequest) that will restart
   the transaction event loop.

 * The TransportInfo that's passed in the same callback carries
   information from an ACCEPT_CH frame up to URLLoader::OnConnected.

And of course, new unit tests and piping everything together.

Bug: 591068, 1168489
Change-Id: I92f1f95e0969793818a8a0b35aaa768032705c31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2690027
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Aaron Tagliaboschi <aarontag@chromium.org>
Cr-Commit-Position: refs/heads/master@{#861732}

• net/http/http_network_transaction.cc[diff]
• net/http/http_network_transaction.h[diff]
• net/http/http_network_transaction_unittest.cc[diff]
• net/http/http_transaction.h[diff]
• net/http/http_transaction_test_util.cc[diff]
• net/http/http_transaction_test_util.h[diff]
• net/url_request/url_request.cc[diff]
• net/url_request/url_request.h[diff]
• net/url_request/url_request_http_job.cc[diff]
• net/url_request/url_request_http_job.h[diff]
• net/url_request/url_request_job.cc[diff]
• net/url_request/url_request_job.h[diff]
• net/url_request/url_request_test_util.cc[diff]
• net/url_request/url_request_test_util.h[diff]
• net/url_request/url_request_unittest.cc[diff]
• services/network/url_loader.cc[diff]
• services/network/url_loader.h[diff]
• services/network/url_loader_unittest.cc[diff]