Previously PortAllocator instances were created asynchronously in
TransportContext. Making PortAllocator synchronous allows to simplify
logic in other places, particularly in WebrtcTransport. Also simplified
PortAllocatorBase - it no longer stores STUN and Relay information and
instead instances of PortAllocatorSessionBase call TransportContext
to request fresh information when allocating ports.

Also removed duplicate logic from PepperPortAllocator and
ChromiumPortAllocator. Specifically they were filtering TCP-based relay
configs, but that's not longer necessary because PortAllocatorBase
is responsible for that now.

BUG=547158

Review URL: https://codereview.chromium.org/1571943002

Cr-Commit-Position: refs/heads/master@{#368701}