Create RtcEventLogEncoderLegacy

Create RtcEventLogEncoderLegacy

We're moving to an RtcEventLog interface that accepts std::unique_ptr<EventLog> and stores the event for encoding when encoding becomes necessary, rather than before. This will be useful while we maintain the legacy (current) encoding alongside the new encoding on which we're working.

This CL introduces RtcEventLogEncoderLegacy, which takes provides the encoding currently done by RtcEventLogImpl. After this, we can modify RtcEventLogImpl to use a dynamically chosen encoding, allowing us to easily choose between the current encoding and the new one on which we're working.

BUG=webrtc:8111

[eladalon, 2017-09-11 17:08:27]

eladalon@webrtc.org changed reviewers:
+ terelius@webrtc.org

[eladalon, 2017-09-11 17:08:28]

PTAL