[fetch-later] Implement aborting a FetchLater request
## Summary ##
(FetchLater is a flag-guarded new API)
Aborting is implemented by propagating a custom mojo disconnection
reason from remote<URLLoader> (renderer) to receiver<URLLoader>
(browser).
Unlike previous approach [1], this CL does not introduce new IPC. So
there should be no IPC sequencing issue.
## Before this CL ##
The `KeepAliveURLLoader` in browser starts all pending requests when
it learns the disconnection of its receiver<URLLoader>.
## After this CL ##
By default, it will still start pending request on disconnection of
receiver<URLLoader>. However, if the disconnection comes with a custom
reason of pair (kClientDisconnectReason,
kFetchLaterDisconnectReasonExplicitlyAborted), the
pending request will be aborted instead.
## Details ##
Every time JS abort() call happens for a FetchLater request,
the following extra flow is executed:
```
FetchManager::Loader::Abort()
->ThreadableLoader::Abort()
->ResourceLoader::AbortFetchLaterRequest()
->ResourceLoader::Cancel()
->(Calls to blink::URLLoader/ResourceRequestSender in between...)
->ResourceRequestSender::DeletePendingRequest()
->ThrottlingURLLoader::CancelWithError(
..., kFetchLaterDisconnectReasonExplicitlyAborted)
```
`ResourceRequestSender` is able to learn that it is deleting an
aborted FetchLater request by asking
`FetchLaterLoaderHelper::IsExplicitlyAborted()` it holds, which is a
new interface implemented by the `ResourceLoader` that creates itself.
The helper is only provided to `ResourceRequestSender::SendAsync()`
when the request is a FetchLater request.
[1]: https://crrev.com/c/4803083/23..29/third_party/blink/renderer/core/fetch/fetch_manager.cc#b1096
Bug: 1465781
Change-Id: I8762c37e1c64b3661d760cfb62f0ea8f02dddf1c
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