Revert of Delete VideoFrame default constructor, and IsZeroSize method.

webrtc/video_frame.h

Index: webrtc/video_frame.h
diff --git a/webrtc/video_frame.h b/webrtc/video_frame.h
index 5fd62a6bbb08e30f41d229468adfaf5e0e3cd97b..a834f83ddf1333c74e3f3f70779e905248aceec2 100644
--- a/webrtc/video_frame.h
+++ b/webrtc/video_frame.h
@@ -22,6 +22,10 @@
 
 class VideoFrame {
  public:
+  // TODO(nisse): Deprecated. Using the default constructor violates the
+  // reasonable assumption that video_frame_buffer() returns a valid buffer.
+  VideoFrame();
+
   // TODO(nisse): This constructor is consistent with
   // cricket::WebRtcVideoFrame. After the class
   // cricket::WebRtcVideoFrame and its baseclass cricket::VideoFrame
@@ -102,6 +106,15 @@
     return timestamp_us() / rtc::kNumMicrosecsPerMillisec;
   }
 
+  // Return true if and only if video_frame_buffer() is null. Which is possible
+  // only if the object was default-constructed.
+  // TODO(nisse): Deprecated. Should be deleted in the cricket::VideoFrame and
+  // webrtc::VideoFrame merge. The intention is that video_frame_buffer() never
+  // should return nullptr. To handle potentially uninitialized or non-existent
+  // frames, consider using rtc::Optional. Otherwise, IsZeroSize() can be
+  // replaced by video_frame_buffer() == nullptr.
+  bool IsZeroSize() const;
+
   // Return the underlying buffer. Never nullptr for a properly
   // initialized VideoFrame.
   rtc::scoped_refptr<webrtc::VideoFrameBuffer> video_frame_buffer() const;