ResourceDispatcher: clarify RequestPeer's ownership

https://goo.gl/WUyPxQ has more detailed problem description and
rough design for this change. In short, this patch does:

- Let each RequestPeer owned by the module that is talking to the peer, i.e.
  ResourceDispatcher owns the first RequestPeer in the chain and each
  RequestPeer owns its next peer in the chain.
- Introduce WebURLLoaderImpl::RequestPeerImpl as a (non-refcounted) RequestPeer.
  instance. RequestPeerImpl owns a ref to WebURLLoaderImpl::Context and it's
  solely owned by ResourceDispatcher (or its previous peer).
- Remove self-protect refptr's in the code but instead make PendingRequestInfo
  always get deleted asyncly, so that it is always guaranteed that it is kept
  alive while the incoming message is being handled.

If this looks fine I'll remove
RequestPeer::OnReceivedCompletedResponse in a follow-up patch
(and add a new test for the new code)

BUG=507170
TEST=existing tests

Review URL: https://codereview.chromium.org/1657483002

Cr-Commit-Position: refs/heads/master@{#373373}

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