Use WeakPtr for RTCPeerConnectionHandler and RTCPeerConnectionTracker.

      These objects don't have the same lifetime and we were using raw pointers to them  in a few places, which could lead to use-after-free.

BUG=429477
TBR=perkj@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/698803002

Cr-Commit-Position: refs/heads/master@{#302398}