The new TransportContext is now used to store all parameters required
to initialize Transport objects and is applicable both to IceTransport
and WebrtcTransport. It also allowed to reduce amount of boilerplate
code when passing around these parameters.

BUG=547158

Review URL: https://codereview.chromium.org/1521883006

Cr-Commit-Position: refs/heads/master@{#365649}