A bunch of interfaces: Return scoped_ptr<SSLCertificate>

Instead of using a raw pointer output parameter. This affects

  SSLStreamAdapter::GetPeerCertificate
  Transport::GetRemoteSSLCertificate
  TransportChannel::GetRemoteSSLCertificate
  TransportController::GetRemoteSSLCertificate
  WebRtcSession::GetRemoteSSLCertificate

This is a good idea in general, but will also be very convenient when
scoped_ptr is gone, since unique_ptr doesn't have an .accept() method.

BUG=webrtc:5520

Review URL: https://codereview.webrtc.org/1802013002

Cr-Commit-Position: refs/heads/master@{#12262}
diff --git a/webrtc/api/webrtcsession.h b/webrtc/api/webrtcsession.h
index 6164731..01ec526 100644
--- a/webrtc/api/webrtcsession.h
+++ b/webrtc/api/webrtcsession.h
@@ -292,8 +292,8 @@
       rtc::scoped_refptr<rtc::RTCCertificate>* certificate);
 
   // Caller owns returned certificate
-  virtual bool GetRemoteSSLCertificate(const std::string& transport_name,
-                                       rtc::SSLCertificate** cert);
+  virtual rtc::scoped_ptr<rtc::SSLCertificate> GetRemoteSSLCertificate(
+      const std::string& transport_name);
 
   cricket::DataChannelType data_channel_type() const;