Make screenshare target bitrate experiment always on

BUG=4083
R=mflodman@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44699004

Patch from sprang@webrtc.org <sprang@webrtc.org>.

Cr-Original-Commit-Position: refs/heads/master@{#8806}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 8904290aca8860cca4efb3a06f1557254b2401fb
3 files changed