Remove encoding code from RtcEventLogImpl and use RtcEventLogEncoder instead

Remove encoding code from RtcEventLogImpl and use RtcEventLogEncoder instead

RtcEventLogImpl no longer hard-codes the way encoding is done. It now relies on RtcEventEncoder for it. This gives two benefits:
1. We can decide between the current encoding and the new encoding (which is still WIP) without code duplication (no need for RtcEventLogImplNew).
2. Encoding is done only when the event needs to be written to a file. This both avoids unnecessary encoding of events which don't end up getting written to a file, as well as is useful for the new, delta-based encoding, which is stateful.

BUG=webrtc:8111

[eladalon, 2017-09-13 15:05:57]

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

[eladalon, 2017-09-13 15:05:58]

PTAL

[terelius, 2017-09-14 08:12:16]

I think this CL should wait until downstream dependencies are updated.

[eladalon, 2017-09-14 08:56:48]

On 2017/09/14 08:12:16, terelius wrote:
> I think this CL should wait until downstream dependencies are updated.

Summarizing offline discussion:
The changes in this CL (and the other CLs in the stack) do not change the output
to the file. We can land this before changing downstream.