Add the TID to the CallStack.

R=ivanpe@chromium.org

Review URL: https://codereview.chromium.org/1849933002 .

src/processor/call_stack.cc

@@ -48,6 +48,7 @@ void CallStack::Clear() {
        ++iterator) {
     delete *iterator;
   }
+  tid_ = 0;
 }
 
 }  // namespace google_breakpad