This change adds HostExperimentSessionPlugin in host ClientSession class, and
attaches it to Session through ConnectionToClient. When OnAuthenticated() is
fired, the HostExperimentSessionPlugin::configuration() will be sent to
HostSesionPlugin / DesktopEnvironmentOptions, so DesktopEnvironment can use it
to create DirectX capturer.

This change is more specific for experiment of DirectX capturer.

BUG=650926

Review-Url: https://codereview.chromium.org/2615113005
Cr-Commit-Position: refs/heads/master@{#448852}