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}

api/video/encoded_image.cc

@@ -52,6 +52,11 @@ size_t EncodedImageBuffer::size() const {
 }
 
 void EncodedImageBuffer::Realloc(size_t size) {
+  // Calling realloc with size == 0 is equivalent to free, and returns nullptr.
+  // Which is confusing on systems where malloc(0) doesn't return a nullptr.
+  // More specifically, it breaks expectations of
+  // VCMSessionInfo::UpdateDataPointers.
+  RTC_DCHECK(size > 0);
   buffer_ = static_cast<uint8_t*>(realloc(buffer_, size));
   size_ = size;
 }