This change adds plugin message in JingleMessage, which is an action indepenent
xml node, placed before or after other messages.

The plugin message will be used to send and receive host attributes and
experiment configuration.

This is part of host experiment framework.

BUG=650926

Review-Url: https://codereview.chromium.org/2567953002
Cr-Commit-Position: refs/heads/master@{#438945}