Add missing windows conditions variable

Those are used by generic RW lock implementation.

https://bugs.freedesktop.org/show_bug.cgi?id=96754

webrtc/system_wrappers/source/condition_variable_event_win.h

@@ -0,0 +1,46 @@
+/*
+ *  Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_SYSTEM_WRAPPERS_SOURCE_CONDITION_VARIABLE_EVENT_WIN_H_
+#define WEBRTC_SYSTEM_WRAPPERS_SOURCE_CONDITION_VARIABLE_EVENT_WIN_H_
+
+#include <windows.h>
+
+#include "webrtc/system_wrappers/include/condition_variable_wrapper.h"
+
+namespace webrtc {
+
+class ConditionVariableEventWin : public ConditionVariableWrapper {
+ public:
+  ConditionVariableEventWin();
+  virtual ~ConditionVariableEventWin();
+
+  void SleepCS(CriticalSectionWrapper& crit_sect);
+  bool SleepCS(CriticalSectionWrapper& crit_sect, unsigned long max_time_inMS);
+  void Wake();
+  void WakeAll();
+
+ private:
+  enum EventWakeUpType {
+    WAKEALL_0   = 0,
+    WAKEALL_1   = 1,
+    WAKE        = 2,
+    EVENT_COUNT = 3
+  };
+
+  unsigned int     num_waiters_[2];
+  EventWakeUpType  eventID_;
+  CRITICAL_SECTION num_waiters_crit_sect_;
+  HANDLE           events_[EVENT_COUNT];
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_SYSTEM_WRAPPERS_SOURCE_CONDITION_VARIABLE_EVENT_WIN_H_