Forward the SignalNetworkRouteChanged from DtlsSrtpTransport to BaseChannel.

In current implementation, the DtlsSrtpTransport listens to the
SignalNetworkRouteChanged but doesn't forward it to the BaseChannel which
makes it impossible for the media engine to update the network route and
the transport overhead.

The BaseChannel unit tests failed to catch this issue because it used a plain
unencrypted RTP transport for testing.

This CL fix that issue and update the BaseChannel tests.

Bug: webrtc:7013, b/73645191
Change-Id: I417b58ff9af4e3c4fac442ff10b5a85bc2093530
Reviewed-on: https://webrtc-review.googlesource.com/55940
Commit-Queue: Zhi Huang <zhihuang@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22140}
diff --git a/pc/channel_unittest.cc b/pc/channel_unittest.cc
index e7a50dc..92d3b24 100644
--- a/pc/channel_unittest.cc
+++ b/pc/channel_unittest.cc
@@ -1030,8 +1030,10 @@
     static constexpr int kLastPacketId = 100;
     // Ipv4(20) + UDP(8).
     static constexpr int kTransportOverheadPerPacket = 28;
+    static constexpr int kSrtpOverheadPerPacket = 10;
 
-    CreateChannels(0, 0);
+    CreateChannels(DTLS, DTLS);
+    SendInitiate();
 
     typename T::MediaChannel* media_channel1 =
         static_cast<typename T::MediaChannel*>(channel1_->media_channel());
@@ -1073,7 +1075,7 @@
     EXPECT_EQ(expected_network_route, media_channel1->last_network_route());
     EXPECT_EQ(kLastPacketId,
               media_channel1->last_network_route().last_sent_packet_id);
-    EXPECT_EQ(kTransportOverheadPerPacket,
+    EXPECT_EQ(kTransportOverheadPerPacket + kSrtpOverheadPerPacket,
               media_channel1->transport_overhead_per_packet());
   }