Introduce an IsMutable method on VideoFrameBuffer.

webrtc/common_video/include/video_frame_buffer.h

Index: webrtc/common_video/include/video_frame_buffer.h
diff --git a/webrtc/common_video/include/video_frame_buffer.h b/webrtc/common_video/include/video_frame_buffer.h
index 9cf57a43591b1da66f82ed9d59ffde5794fdc212..f1b78492baf2f9cadaecb6a243c33cf5460229b0 100644
--- a/webrtc/common_video/include/video_frame_buffer.h
+++ b/webrtc/common_video/include/video_frame_buffer.h
@@ -33,7 +33,13 @@ enum PlaneType {
 // not contain any frame metadata such as rotation, timestamp, pixel_width, etc.
 class VideoFrameBuffer : public rtc::RefCountInterface {
  public:
-  // Returns true if this buffer has a single exclusive owner.
+  // Returns true if the caller is exclusive owner, and allowed to
+  // call MutableData.
+  virtual bool IsMutable() = 0;
+
+  // Underlying refcount access, used to implement IsMutable.
+  // TODO(nisse): Demote to protected, as soon as Chrome is changed to
+  // use IsMutable.
   virtual bool HasOneRef() const = 0;
 
   // The resolution of the frame in pixels. For formats where some planes are
@@ -76,6 +82,7 @@ class I420Buffer : public VideoFrameBuffer {
   const uint8_t* data(PlaneType type) const override;
   // Non-const data access is only allowed if HasOneRef() is true to protect
   // against unexpected overwrites.
+  bool IsMutable() override;
   uint8_t* MutableData(PlaneType type) override;
   int stride(PlaneType type) const override;
   void* native_handle() const override;
@@ -110,6 +117,7 @@ class NativeHandleBuffer : public VideoFrameBuffer {
   const uint8_t* data(PlaneType type) const override;
   int stride(PlaneType type) const override;
   void* native_handle() const override;
+  bool IsMutable() override;
 
  protected:
   void* native_handle_;
@@ -131,6 +139,8 @@ class WrappedI420Buffer : public webrtc::VideoFrameBuffer {
   int width() const override;
   int height() const override;
 
+  bool IsMutable() override;
+
   const uint8_t* data(PlaneType type) const override;
 
   int stride(PlaneType type) const override;