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Now XMPP is implemented directly in remoting/signaling instead of
relying on WebRTC. The new code is mostly just a translation of
the XMPP implementation we use in the webapp from JavaScript to
C++ and uses the same layout and class names, except that the main
class is called XmppSignalStrategy instead of XmppConnection.

BUG=459756, 443806, 417336

Review URL: https://codereview.chromium.org/958703003

Cr-Commit-Position: refs/heads/master@{#319712}