Replace RTC_DCHECK(false) with RTC_NOTREACHED().

Bulk of changes done using

  git grep -l 'RTC_DCHECK(false)' | \
    xargs sed -i 's/RTC_DCHECK(false)/RTC_NOTREACHED()/'

peerconnection.cc also used RTC_DCHECK(false && "msg") in two places,
which were updated manually.

BUG=webrtc:6424

Review-Url: https://codereview.webrtc.org/2623313004
Cr-Commit-Position: refs/heads/master@{#16026}
diff --git a/webrtc/p2p/base/dtlstransportchannel.cc b/webrtc/p2p/base/dtlstransportchannel.cc
index 8aea40c..02f3d59 100644
--- a/webrtc/p2p/base/dtlstransportchannel.cc
+++ b/webrtc/p2p/base/dtlstransportchannel.cc
@@ -633,7 +633,7 @@
       // packets in this state, the incoming queue must be empty. We
       // ignore write errors, thus any errors must be because of
       // configuration and therefore are our fault.
-      RTC_DCHECK(false) << "StartSSL failed.";
+      RTC_NOTREACHED() << "StartSSL failed.";
       LOG_J(LS_ERROR, this) << "Couldn't start DTLS handshake";
       set_dtls_state(DTLS_TRANSPORT_FAILED);
       return;