Add a missing const to an accessor.

src/google_breakpad/processor/call_stack.h

Index: src/google_breakpad/processor/call_stack.h
diff --git a/src/google_breakpad/processor/call_stack.h b/src/google_breakpad/processor/call_stack.h
index 7c4a06f57ea5fd0007d356c7ff36b8acd82318f7..c59142315b9a777f383950e541a62ca0405bbadd 100644
--- a/src/google_breakpad/processor/call_stack.h
+++ b/src/google_breakpad/processor/call_stack.h
@@ -68,7 +68,7 @@ class CallStack {
   // 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.