Capability Delegation: match payment-request identifier with spec.

The proposed spec [1] specifies a capability through the name defined
for Permissions Policy [2].  This CL makes the implementation match the
proposal.

[1] https://wicg.github.io/capability-delegation/spec.html
[2] https://github.com/w3c/webappsec-permissions-policy/blob/main/features.md

Bug: 1130558
Change-Id: I926aa030603cfeea10c71fc19f2b232c0d6a5702
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3372328


Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Mustaq Ahmed <mustaq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#957608}