Include more information in --prof log.

src/log.h

Index: src/log.h
diff --git a/src/log.h b/src/log.h
index c8f81bc57547bed3b710494cd6075e8e5f31cec8..cec4e476d7dac283c3707c4f4bad97ee7c4167f9 100644
--- a/src/log.h
+++ b/src/log.h
@@ -275,7 +275,10 @@ class Logger {
   void SharedLibraryEvent(const wchar_t* library_path,
                           uintptr_t start,
                           uintptr_t end);
+  
+  // ==== Events logged by --log-timer-events. ====
   void TimerEvent(const char* name, int64_t start, int64_t end);
+  void ExternalSwitch(StateTag old_tag, StateTag new_tag);
 
   class TimerEventScope {
    public:
@@ -476,6 +479,7 @@ class Logger {
   Address prev_code_;
 
   int64_t epoch_;
+  static int64_t enter_external_;
 
   friend class CpuProfiler;
 };