Implement framesEncoded stat in video send ssrc stats.

Implemented as defined by this pull request: https://github.com/w3c/webrtc-stats/pull/70

BUG=webrtc:6541

Review-Url: https://codereview.webrtc.org/2421193003
Cr-Original-Commit-Position: refs/heads/master@{#14734}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 43536c3d6a728407adbaabc5b2f2d7a42234234a
10 files changed