Revert "Remove default encoder/decoders."

This reverts commit 78ae00eea29850bd3bd608287d8b057c88e91b42 due to perf
regressions. Reverting during investigation to figure out root causes.

BUG=chromium:491112
R=henrik.lundin@webrtc.org
TBR=mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/56449004

Cr-Commit-Position: refs/heads/master@{#9283}
10 files changed