DxgiOutputDuplicator should initialize num_frames_captured_

webrtc/modules/desktop_capture/win/dxgi_output_duplicator.h

 /*
  *  Copyright (c) 2016 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.
  */
 
@@ skipping 110 matching lines @@
   // change this time. And during next Duplicate() function call, their
   // updated_region_ will be merged and copied.
   std::vector<Context*> contexts_;
 
   // The last full frame of this output and its offset. If on AcquireNextFrame()
   // failed because of timeout, i.e. no update, we can copy content from
   // |last_frame_|.
   std::unique_ptr<SharedDesktopFrame> last_frame_;
   DesktopVector last_frame_offset_;
 
-  int64_t num_frames_captured_;
+  int64_t num_frames_captured_ = 0;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_DESKTOP_CAPTURE_WIN_DXGI_OUTPUT_DUPLICATOR_H_