See cl/162492371 for the equivalent experiment-adding change in the client.

There are two main additions in this change:
1. Addition of a host-side --enable-h264 flag, passed to the host, which
    (in the case of chromotocol) ensures that H264 cannot be removed by 
    removing it from the list of possible codecs if both client and host
    do not have it enabled. In the case of webrtc, this flag does not 
    actually do anything; H264 will always be advertised as an available
    codec, but will only be chosen if the experiment is enabled on the
    client. In addition, we make sure the flag is copied in the case of 
    Windows.
2. Addition configuration changes related to this new flag. Namely,
    addition of set_h264_experiment_enabled in CandidateSessionConfig,
    and making sure it is called in the appropriate locations. 

Change-Id: Ic04bf36fb658c7ea8fe770d17954833e3db2a5a2
Reviewed-on: https://chromium-review.googlesource.com/562361


Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Gus Smith <gusss@google.com>
Cr-Commit-Position: refs/heads/master@{#489055}