[Synchronization] Fix a crash in WaitableEventWatcher

base/win/object_watcher.h

Index: base/win/object_watcher.h
diff --git a/base/win/object_watcher.h b/base/win/object_watcher.h
index 67a1e26c76cd1665b700f896075777d4c66abe58..6fddc588dec715c0c17981ae6037697bb08fcc0a 100644
--- a/base/win/object_watcher.h
+++ b/base/win/object_watcher.h
@@ -26,6 +26,10 @@ namespace win {
 // UnregisterWaitEx is automatically called, and any in-flight notification is
 // suppressed.
 //
+// The waiting handle MUST NOT be closed while watching is in progress. If this
+// handle is closed while the wait is still pending, the behavior is undefined
+// (see MSDN:RegisterWaitForSingleObject).
+//
 // Typical usage:
 //
 //   class MyClass : public base::win::ObjectWatcher::Delegate {