webauthn: check that a request is still current after TPM (and other) checks.

The linked bug is a crash on nullptr caused when checking for a TPM
races with the WebAuthn request getting canceled. Unlike many other
callbacks into AuthenticatorCommonImpl, the TPM check will still resolve
even after the request state is destroyed. Thus it needs to know whether
the request that caused it has disappeared from under it.

Normally this would be handled by a WeakPtr, but AuthenticatorCommonImpl
handles multiple requests. Thus we need an identifier for requests so
that callbacks can check whether they're still pertinent. This change
adds a `RequestKey` for this purpose and liberally scatters it around.

It's possible that some of the callbacks where a RequestKey has been
added don't suffer this problem, but a RequestKey is cheap and I added
it rather than investigate too deeply whether it was always needed.

Fixed: 352532554
Change-Id: I59df2fe85e0c464bc094ad01e4fb8c8dceffe22b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5763162
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Auto-Submit: Adam Langley <agl@chromium.org>
Commit-Queue: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1337825}