Takes functionality once in the WebAuthentication DOM class that needs
to be handled by the content process, and moves it to a
singleton (per-content-process) manager class. This allows the
WebAuthn API to centralize management of transactions and IPC
channels. Patch also creates the child (content-process) classes for
WebAuthn IPC channels.
MozReview-Commit-ID: 6ju2LK8lvNR
Kyle Machulis