Add UseCounters for proxied Cryptotoken calls

This adds UseCounters to monitor usage prevalence of the deprecated
u2f.sign and u2f.register methods, as they are proxied to the
corresponding WebAuthn method implementations.

Bug: 907581
Change-Id: I11286be5efd113c6784012bfae70b284946926fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1500027
Reviewed-by: Adam Langley <agl@chromium.org>
Commit-Queue: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637489}
3 files changed