Add a missing const to an accessor.

src/google_breakpad/processor/call_stack.h

 // Copyright (c) 2006, Google Inc.
 // All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
@@ skipping 50 matching lines @@
   ~CallStack();
 
   // Resets the CallStack to its initial empty state
   void Clear();
 
   const vector<StackFrame*>* frames() const { return &frames_; }
 
   // Set the TID associated with this call stack.
   void set_tid(uint32_t tid) { tid_ = tid; }
 
-  uint32_t tid() { return tid_; }
+  uint32_t tid() const { return tid_; }
 
  private:
   // Stackwalker is responsible for building the frames_ vector.
   friend class Stackwalker;
 
   // Storage for pushed frames.
   vector<StackFrame*> frames_;
 
   // The TID associated with this call stack. Default to 0 if it's not
   // available.
   uint32_t tid_;
 };
 
 }  // namespace google_breakpad
 
 #endif  // GOOGLE_BREAKPAD_PROCSSOR_CALL_STACK_H__