Reason for revert:
Previous mode aligns with other encoders, and RC_TIMESTAMP_MODE might have issues with no frames for several seconds.
Original issue's description:
> Use RC_TIMESTAMP_MODE for OpenH264.
>
> Performs rate control based on timestamp deltas instead of announced
> frame rate.
>
> BUG=webrtc:5855
> R=hbos@webrtc.org
>
> Committed: https://crrev.com/c4deee49a3ec42b7fe83c82f750539b36aae1d3f
> Cr-Commit-Position: refs/heads/master@{#12611}
TBR=hbos@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:5855
Review-Url: https://codereview.webrtc.org/1950973002
Cr-Commit-Position: refs/heads/master@{#12629}
b49ac78c71