Fix a possible mismatch in OS-specific code in WaitableEventWatcher.

base/synchronization/waitable_event_watcher.h

Index: base/synchronization/waitable_event_watcher.h
diff --git a/base/synchronization/waitable_event_watcher.h b/base/synchronization/waitable_event_watcher.h
index 966a468b1235b44f4ece0d9bbc1ecbc0afc4aed4..1512acc87250f45918bb3158e97636d79b660b22 100644
--- a/base/synchronization/waitable_event_watcher.h
+++ b/base/synchronization/waitable_event_watcher.h
@@ -6,6 +6,7 @@
 #define BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_
 #pragma once
 
+#include "base/base_export.h"
 #include "build/build_config.h"
 
 #if defined(OS_WIN)
@@ -16,8 +17,6 @@
 #include "base/synchronization/waitable_event.h"
 #endif
 
-#include "base/base_export.h"
-
 namespace base {
 
 class Flag;
@@ -62,7 +61,7 @@ class WaitableEvent;
 // -----------------------------------------------------------------------------
 
 class BASE_EXPORT WaitableEventWatcher
-#if defined(OS_POSIX)
+#if !defined(OS_WIN)
     : public MessageLoop::DestructionObserver
 #endif
 {