Refactor legacy FrameBuffer to use EncodedImageBuffer::Realloc

Preparation for deleting VCMEncodedFrame::VerifyAndAllocate and
EncodedImage::Allocate.

Bug: webrtc:9378
Change-Id: If7c16061962bbd58c3e7d5720189854e00a3d7bf
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/154570
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29339}

modules/video_coding/frame_buffer.h

@@ -76,6 +76,9 @@ class VCMFrameBuffer : public VCMEncodedFrame {
   void SetState(VCMFrameBufferStateEnum state);  // Set state of frame
 
   VCMFrameBufferStateEnum _state;  // Current state of the frame
+  // Set with SetEncodedData, but keep pointer to the concrete class here, to
+  // enable reallocation and mutation.
+  rtc::scoped_refptr<EncodedImageBuffer> encoded_image_buffer_;
   VCMSessionInfo _sessionInfo;
   uint16_t _nackCount;
   int64_t _latestPacketTimeMs;