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FIDO U2F defines initialization packets and continuation packets.
This adds class definitions for those packets, and for an
enclosing message class. The enclosing message class constructs
the initialization and continuation packets based on the payload
length, queueing them in a list format for consumption by a
device connection.

BUG=NONE

Review-Url: https://codereview.chromium.org/2502103002
Cr-Commit-Position: refs/heads/master@{#438925}