Move the ICE state transition ASSERTS to a lower level.

webrtc/p2p/base/p2ptransportchannel.cc

Index: webrtc/p2p/base/p2ptransportchannel.cc
diff --git a/webrtc/p2p/base/p2ptransportchannel.cc b/webrtc/p2p/base/p2ptransportchannel.cc
index 2801c446461679011e091ccd5fd5af46dcb81cdf..0294f1b41154134e8af990994810aec4b238c7ce 100644
--- a/webrtc/p2p/base/p2ptransportchannel.cc
+++ b/webrtc/p2p/base/p2ptransportchannel.cc
@@ -1209,8 +1209,43 @@ void P2PTransportChannel::SwitchBestConnectionTo(Connection* conn) {
 // change, it should be called after all the connection states have changed. For
 // example, we call this at the end of SortConnections.
 void P2PTransportChannel::UpdateState() {
+  TransportChannelState old_state = state_;
   state_ = ComputeState();
 
+  if (old_state != state_) {
+    LOG_J(LS_INFO, this) << "Changing TransportChannelState " << old_state
+                         << " => " << state_;
+
+    // Check that the requested transition is allowed. Note that
+    // P2PTransportChannel does not (yet) implement a direct mapping of the ICE
+    // states from the standard; the difference is covered by
+    // TransportController and PeerConnection.
+    switch (old_state) {
+      case STATE_INIT:
+        // TODO(deadbeef): Once we implement end-of-candidates signaling,
+        // we shouldn't go from INIT to COMPLETED.
+        RTC_DCHECK(state_ == STATE_CONNECTING || state_ == STATE_COMPLETED);
+        break;
+      case STATE_CONNECTING:
+        RTC_DCHECK(state_ == STATE_COMPLETED || state_ == STATE_FAILED);
+        break;
+      case STATE_COMPLETED:
+        // TODO(deadbeef): Once we implement end-of-candidates signaling,
+        // we shouldn't go from COMPLETED to CONNECTING.
+        // Though we *can* go from COMPlETED to FAILED, if consent expires.
+        RTC_DCHECK(state_ == STATE_CONNECTING || state_ == STATE_FAILED);
+        break;
+      case STATE_FAILED:
+        // TODO(deadbeef): Once we implement end-of-candidates signaling,
+        // we shouldn't go from FAILED to CONNECTING or COMPLETED.
+        RTC_DCHECK(state_ == STATE_CONNECTING || state_ == STATE_COMPLETED);
+        break;
+      default:
+        RTC_DCHECK(false);
+        break;
+    }
+  }
+
   bool writable = best_connection_ && best_connection_->writable();
   set_writable(writable);