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This adds the client plumbing needed to get an arbitrary JSON message from client to host, or vice versa. This will allow us to extend the protocol at short notice, without needing to wait for client plugin changes to be promoted to Stable.

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Review URL: https://chromiumcodereview.appspot.com/22477006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@217127 0039d316-1c4b-4281-b951-d872f2087c98