Refactoring: Declare cricket::Codec constructors protected.

This makes it obvious that cricket::Codec should not be
instantiated; only subclasses should be instantiated.

BUG=none

Review-Url: https://codereview.webrtc.org/2546363002
Cr-Commit-Position: refs/heads/master@{#15468}
diff --git a/webrtc/api/webrtcsdp.cc b/webrtc/api/webrtcsdp.cc
index d78112c..69a1af2 100644
--- a/webrtc/api/webrtcsdp.cc
+++ b/webrtc/api/webrtcsdp.cc
@@ -2385,8 +2385,7 @@
   // Codec has not been populated correctly unless the name has been set. This
   // can happen if an SDP has an fmtp or rtcp-fb with a payload type but doesn't
   // have a corresponding "rtpmap" line.
-  cricket::Codec default_codec;
-  return default_codec.name != codec.name;
+  return !codec.name.empty();
 }
 
 bool VerifyAudioCodecs(const AudioContentDescription* audio_desc) {