Change DtmfSenderHandler to handle events on the signaling thread.
Since the RTCPeerConnectionHandler now uses a separate thread for API-calls to libjingle (https://codereview.chromium.org/
675013005/) the DtmfSenderHandler must also handle events on the signaling thread.
This cl also fix a problem where a remote video source falsly DCHECKs that an event have been missed.
TEST=out/Debug/content_browsertests --gtest_filter=WebRtcBrowserTest.MANU* --run-manual
BUG=369796
Review URL: https://codereview.chromium.org/
699613002
Cr-Commit-Position: refs/heads/master@{#302439}