Allow TransportController to create a QuicTransportChannel

A QuicTransport is implemented that subclasses Transport
and takes ownership of the QuicTransportChannel/P2PTransportChannel.

Split from CL https://codereview.webrtc.org/1844803002/.

BUG=

Review-Url: https://codereview.webrtc.org/1856943002
Cr-Commit-Position: refs/heads/master@{#12575}
diff --git a/webrtc/base/sslfingerprint.cc b/webrtc/base/sslfingerprint.cc
index 1939b4f..2c3e1e9 100644
--- a/webrtc/base/sslfingerprint.cc
+++ b/webrtc/base/sslfingerprint.cc
@@ -85,7 +85,7 @@
   return fingerprint;
 }
 
-std::string SSLFingerprint::ToString() {
+std::string SSLFingerprint::ToString() const {
   std::string fp_str = algorithm;
   fp_str.append(" ");
   fp_str.append(GetRfc4572Fingerprint());
diff --git a/webrtc/base/sslfingerprint.h b/webrtc/base/sslfingerprint.h
index 1413a4c..4ffb2b0 100644
--- a/webrtc/base/sslfingerprint.h
+++ b/webrtc/base/sslfingerprint.h
@@ -41,7 +41,7 @@
 
   std::string GetRfc4572Fingerprint() const;
 
-  std::string ToString();
+  std::string ToString() const;
 
   std::string algorithm;
   rtc::CopyOnWriteBuffer digest;