Waterfall to use run-time flag WebRTC-H264WithOpenH264FFmpeg.

Updated fieldtrial_testing_config file for chromeos, linux,
mac, win. (Feature not supported on android/ios.)

Note: The run-time flag was defined in:
https://codereview.chromium.org/1718883002/.

BUG=chromium:500605, chromium:468365

Review URL: https://codereview.chromium.org/1696383002

Cr-Commit-Position: refs/heads/master@{#378424}
4 files changed