Loosening the coupling between WebRTC and //third_party/protobuf

webrtc/logging/rtc_event_log/rtc_event_log_helper_thread.h

Index: webrtc/logging/rtc_event_log/rtc_event_log_helper_thread.h
diff --git a/webrtc/logging/rtc_event_log/rtc_event_log_helper_thread.h b/webrtc/logging/rtc_event_log/rtc_event_log_helper_thread.h
index 420e5c529f97db3999eddca98c5bea4b46efcf40..4befcdb9b76bf4ca46a6456774f40b13691c8546 100644
--- a/webrtc/logging/rtc_event_log/rtc_event_log_helper_thread.h
+++ b/webrtc/logging/rtc_event_log/rtc_event_log_helper_thread.h
@@ -13,7 +13,6 @@
 
 #include <limits>
 #include <memory>
-#include <string>
 #include <utility>
 #include <vector>
 
@@ -21,6 +20,7 @@
 #include "webrtc/base/event.h"
 #include "webrtc/base/ignore_wundef.h"
 #include "webrtc/base/platform_thread.h"
+#include "webrtc/base/protobuf_utils.h"
 #include "webrtc/base/swap_queue.h"
 #include "webrtc/logging/rtc_event_log/ringbuffer.h"
 #include "webrtc/system_wrappers/include/file_wrapper.h"
@@ -112,7 +112,7 @@ class RtcEventLogHelperThread final {
   std::unique_ptr<rtclog::Event> most_recent_event_;
 
   // Temporary space for serializing profobuf data.
-  std::string output_string_;
+  ProtoString output_string_;
 
   rtc::Event wake_periodically_;
   rtc::Event wake_from_hibernation_;