Adding DCHECKs and constness to DtlsIdentityStore.

talk/app/webrtc/dtlsidentitystore.h

 /*
  * libjingle
  * Copyright 2015 Google Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
  *
  *  1. Redistributions of source code must retain the above copyright notice,
  *     this list of conditions and the following disclaimer.
  *  2. Redistributions in binary form must reproduce the above copyright notice,
@@ skipping 61 matching lines @@
   class WorkerTask;
   typedef rtc::ScopedMessageData<DtlsIdentityStore::WorkerTask>
       IdentityTaskMessageData;
 
   void GenerateIdentity();
   void OnIdentityGenerated(rtc::scoped_ptr<rtc::SSLIdentity> identity);
   void ReturnIdentity(rtc::scoped_ptr<rtc::SSLIdentity> identity);
 
   void PostGenerateIdentityResult_w(rtc::scoped_ptr<rtc::SSLIdentity> identity);
 
-  rtc::Thread* signaling_thread_;
-  rtc::Thread* worker_thread_;
+  rtc::Thread* const signaling_thread_;
+  rtc::Thread* const worker_thread_;
 
   // These members should be accessed on the signaling thread only.
   int pending_jobs_;
   rtc::scoped_ptr<rtc::SSLIdentity> free_identity_;
   typedef std::queue<rtc::scoped_refptr<webrtc::DTLSIdentityRequestObserver>>
       ObserverList;
   ObserverList pending_observers_;
 };
 
 }  // namespace webrtc
 
 #endif  // TALK_APP_WEBRTC_DTLSIDENTITYSTORE_H_