[Synchronization] Fix a crash in WaitableEventWatcher

base/synchronization/waitable_event.h

Index: base/synchronization/waitable_event.h
diff --git a/base/synchronization/waitable_event.h b/base/synchronization/waitable_event.h
index e8caffeec32d5b7418a87ed563f9a86eaf0c92ce..716115cc777dd06822566b18c6235e61e6d41c9d 100644
--- a/base/synchronization/waitable_event.h
+++ b/base/synchronization/waitable_event.h
@@ -155,9 +155,12 @@ class BASE_EXPORT WaitableEvent {
 #if defined(OS_WIN)
   win::ScopedHandle handle_;
 #else
-  // On Windows, one can close a HANDLE which is currently being waited on. The
-  // MSDN documentation says that the resulting behaviour is 'undefined', but
-  // it doesn't crash. However, if we were to include the following members
+  // On Windows, you must not close a HANDLE which is currently being waited on.
+  // The MSDN documentation says that the resulting behaviour is 'undefined'.
+  // To solve that issue each WaitableEventWatcher duplicates the given event
+  // handle.
+
+  // However, if we were to include the following members
   // directly then, on POSIX, one couldn't use WaitableEventWatcher to watch an
   // event which gets deleted. This mismatch has bitten us several times now,
   // so we have a kernel of the WaitableEvent, which is reference counted.