Add U2F request state machines

U2fRequest base class is extended by each type of U2F
request. Common functions of enumeration and device iteration
are defined in the base class. U2fSign performs an entire
sign request on any attached U2F devices, and U2FRegister
performs the registration request on the devices.

BUG=686310

Review-Url: https://codereview.chromium.org/2821263005
Cr-Commit-Position: refs/heads/master@{#466529}
17 files changed