Remove final from rtc::Buffer.

With it removed, you can now use it with scoped_refptr by wrapping it in
an rtc::RefCountedObject<rtc::Buffer>.

BUG=

Review URL: https://codereview.webrtc.org/1414053003

Cr-Commit-Position: refs/heads/master@{#10386}

webrtc/base/buffer.h

@@ -44,7 +44,7 @@ struct ByteType {
 
 // Basic buffer class, can be grown and shrunk dynamically.
 // Unlike std::string/vector, does not initialize data when expanding capacity.
-class Buffer final {
+class Buffer {
  public:
   Buffer();                   // An empty buffer.
   Buffer(const Buffer& buf);  // Copy size and contents of an existing buffer.