Use explicit conversion to base::Optional in MSVS::GetCurrentFormatImpl implementations.

content/renderer/media/mock_media_stream_video_source.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/renderer/media/mock_media_stream_video_source.h"
 
 #include "base/bind.h"
 #include "base/callback_helpers.h"
 #include "base/location.h"
 
@@ skipping 86 matching lines @@
 
   attempted_to_start_ = true;
   frame_callback_ = frame_callback;
 }
 
 void MockMediaStreamVideoSource::StopSourceImpl() {
 }
 
 base::Optional<media::VideoCaptureFormat>
 MockMediaStreamVideoSource::GetCurrentFormatImpl() const {
-  return format_;
+  return base::Optional<media::VideoCaptureFormat>(format_);
 }
 
 void MockMediaStreamVideoSource::DeliverVideoFrame(
     const scoped_refptr<media::VideoFrame>& frame) {
   DCHECK(!frame_callback_.is_null());
   io_task_runner()->PostTask(
       FROM_HERE, base::Bind(frame_callback_, frame, base::TimeTicks()));
 }
 
 }  // namespace content