VideoCaptureDevice: refactor Client::Buffer into an interface

media/video/capture/video_capture_device.h

Index: media/video/capture/video_capture_device.h
diff --git a/media/video/capture/video_capture_device.h b/media/video/capture/video_capture_device.h
index d03b185cee0cf56bc514eedca7ba25064f92fcc1..84a211f692bbb314c064debab6f9d29dcb8dfac1 100644
--- a/media/video/capture/video_capture_device.h
+++ b/media/video/capture/video_capture_device.h
@@ -175,20 +175,13 @@ class MEDIA_EXPORT VideoCaptureDevice {
     // Memory buffer returned by Client::ReserveOutputBuffer().
     class Buffer : public base::RefCountedThreadSafe<Buffer> {
      public:
-      int id() const { return id_; }
-      void* data() const { return data_; }
-      size_t size() const { return size_; }
+      virtual int id() const = 0;
+      virtual void* data() const = 0;
+      virtual size_t size() const = 0;
 
      protected:
       friend class base::RefCountedThreadSafe<Buffer>;
-
-      Buffer(int id, void* data, size_t size)
-          : id_(id), data_(data), size_(size) {}
       virtual ~Buffer() {}
-
-      const int id_;
-      void* const data_;
-      const size_t size_;
     };
 
     virtual ~Client() {}