Let PacketSource::NextPacket() return an std::unique_ptr
The return type of PacketSource::NextPacket() is changed from a naked
pointer to an std::uniqe_ptr. The interface contract was and still is
that the ownership is passed from the callee to the caller, but a
unique_ptr makes this explicit.
BUG=
webrtc:2692
Committed:
https://crrev.com/46ba49c622d305dff756d4d228d9ec953a2aea4e
Cr-Commit-Position: refs/heads/master@{#12884}