Trust Tokens: Give locally fulfilled issuance a distinct exception

A Trust Tokens issuance request is a request created by
  fetch(..., {trustToken: {type: 'token-request'}})

With the new "platform-provided trust tokens" expansion of the Trust
Tokens feature, the request's destination origin can provide
configuration indicating that the browser should divert the origin's
Trust Tokens issuance requests to system-local providers, on platforms
that support this behavior (currently Android). [1]

In this case, there's no need to send an HTTP request directly to the
destination URL provided in the fetch() call, so we return early via
URLLoader::OnComplete(URLLoaderCompletionStatus). We provide a
non-net::OK error code to this URLLoader::OnComplete call since we won't
receive an HTTP response from the request's destination.

Since this case is semantically a success, we want the caller to be able
to distinguish it from errors encountered during the Trust Tokens
operation. This CL updates trust_token_to_mojom.cc to throw a
NoModificationAllowedError, instead of OperationError, when operations
are successfully diverted locally.

It might make more sense from an API design perspective to resolve the
promise, instead of rejecting it, in this case. This would require some
refactoring in the loader (or the willingness to resolve a promise from
FetchManager::Loader::DidFail!). For now, returning a distinct exception
at least allows callers to distinguish this success case from the
feature's failure cases.

I've updated the user guide to mention this behavior. [2]

[1]: https://bit.ly/platform-provided-trust-tokens
[2]: https://bit.ly/trust-token-prototype-user-guide

R=yhirano
CC=zhengwe@google.com

Test: The child CL adds browser tests covering this path
Bug: 1154847
Change-Id: Ie56ddd0aaf544289184f7d0b47f525fcb654877b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2585792
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838238}

third_party/blink/renderer/core/fetch/trust_token_to_mojom.cc

@@ -112,6 +112,13 @@ DOMException* TrustTokenErrorToDOMException(
           "cache hit",
           DOMException::GetErrorName(
               DOMExceptionCode::kNoModificationAllowedError));
+    case network::mojom::blink::TrustTokenOperationStatus::
+        kOperationSuccessfullyFulfilledLocally:
+      return DOMException::Create(
+          "Trust Tokens operation satisfied locally, without needing to send "
+          "the request to its initial destination",
+          DOMException::GetErrorName(
+              DOMExceptionCode::kNoModificationAllowedError));
     case network::mojom::blink::TrustTokenOperationStatus::kFailedPrecondition:
       return DOMException::Create(
           "Precondition failed during Trust Tokens operation",