[webauthn] Add histogram for GPM onboarding flow

The onboarding flow starts when the Google Password Manager option is
selected (`GPMEnclaveController::OnGPMSelected` is called) and the
enclave state is "empty". A boolean (`in_onboarding_flow`) is stored in
`AuthenticatorRequestDialogModel` to allow checking if particular modals
are shown as part of the onboarding flow.

Fixed: 383080284
Change-Id: I8f72a617343ae87e94803ae274b2335ccfefcbf3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6080391
Reviewed-by: Adem Derinel <derinel@google.com>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Andrii Natiahlyi <natiahlyi@google.com>
Cr-Commit-Position: refs/heads/main@{#1393673}