This patch fixes two things:
1) A potential threading issue in setting and reading the PrincipalHandle for
   MediaPipelineReceiveVideo.
2) Plumbs the PrincipalHandle down to the receiving MediaPipelines right from
   the start. It hasn't been necessary in the past as initially the principal
   handed to a track's constructor is trusted, but it's good form and will help
   clarify the situation for the next patch which switches the initial
   principal from always-private to mostly-non-private. In most cases this
   principal does no longer get updated after the track's been created, so it
   helps to have a PrincipalHandle that matches the track's principal.

Differential Revision: https://phabricator.services.mozilla.com/D48946

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extra : moz-landing-system : lando