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/include/condition_variable_wrapper.h

@@ -0,0 +1,42 @@
+/*
+ *  Copyright (c) 2011 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_INCLUDE_CONDITION_VARIABLE_WRAPPER_H_
+#define WEBRTC_SYSTEM_WRAPPERS_INCLUDE_CONDITION_VARIABLE_WRAPPER_H_
+
+namespace webrtc {
+
+class CriticalSectionWrapper;
+
+class ConditionVariableWrapper {
+ public:
+  // Factory method, constructor disabled.
+  static ConditionVariableWrapper* CreateConditionVariable();
+
+  virtual ~ConditionVariableWrapper() {}
+
+  // Calling thread will atomically release crit_sect and wait until next
+  // some other thread calls Wake() or WakeAll().
+  virtual void SleepCS(CriticalSectionWrapper& crit_sect) = 0;
+
+  // Same as above but with a timeout.
+  virtual bool SleepCS(CriticalSectionWrapper& crit_sect,
+                       unsigned long max_time_in_ms) = 0;
+
+  // Wakes one thread calling SleepCS().
+  virtual void Wake() = 0;
+
+  // Wakes all threads calling SleepCS().
+  virtual void WakeAll() = 0;
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_SYSTEM_WRAPPERS_INCLUDE_CONDITION_VARIABLE_WRAPPER_H_