Log stack trace when ObjectWatcher::StartWatching() is called.

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 2a515a19cb98ab9f7ee7491cca81fc870f2f52bc..51ae31a386ea15bcbae98936539698c03c0d8d9f 100644
--- a/base/win/object_watcher.h
+++ b/base/win/object_watcher.h
@@ -10,6 +10,7 @@
 
 #include "base/base_export.h"
 #include "base/callback.h"
+#include "base/debug/stack_trace.h"
 #include "base/memory/weak_ptr.h"
 #include "base/message_loop.h"
 
@@ -93,6 +94,9 @@ class BASE_EXPORT ObjectWatcher : public MessageLoop::DestructionObserver {
   HANDLE wait_object_;        // Returned by RegisterWaitForSingleObject
   MessageLoop* origin_loop_;  // Used to get back to the origin thread
 
+  // For debugging crbug.com/121086.
+  base::debug::StackTrace stack_trace_;
+
   DISALLOW_COPY_AND_ASSIGN(ObjectWatcher);
 };