Revert of Use CopyOnWriteBuffer instead of Buffer to avoid unnecessary copies. (patchset #4 id:60001 of https://codereview.webrtc.org/1785713005/ )

Reason for revert:
I'm really sorry for having to revert this but it seems this hit an unexpected compile error downstream:

webrtc/media/sctp/sctpdataengine.cc: In function 'void cricket::VerboseLogPacket(const void*, size_t, int)':
webrtc/media/sctp/sctpdataengine.cc:172:37: error: invalid conversion from 'const void*' to 'void*' [-fpermissive]
              data, length, direction)) != NULL) {
                                     ^
In file included from webrtc/media/sctp/sctpdataengine.cc:20:0:
third_party/usrsctp/usrsctplib/usrsctp.h:964:1: error:   initializing argument 1 of 'char* usrsctp_dumppacket(void*, size_t, int)' [-fpermissive]
 usrsctp_dumppacket(void *, size_t, int);
 ^

I'm sure you can fix this easily and just re-land this CL, while I'm going to look into how to add this warning at the public bots (on Monday).

Original issue's description:
> Use CopyOnWriteBuffer instead of Buffer to avoid unnecessary copies.
>
> This CL removes copy and assign support from Buffer and changes various
> parameters from Buffer to CopyOnWriteBuffer so they can be passed along
> and copied without actually copying the underlying data.
>
> With this changed some parameters to be "const" and fixed an issue when
> creating a CopyOnWriteBuffer with empty data.
>
> BUG=webrtc:5155
>
> Committed: https://crrev.com/944c39006f1c52aee20919676002dac7a42b1c05
> Cr-Commit-Position: refs/heads/master@{#12058}

TBR=kwiberg@webrtc.org,tkchin@webrtc.org,tommi@webrtc.org,pthatcher@webrtc.org,jbauch@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:5155

Review URL: https://codereview.webrtc.org/1817753003

Cr-Commit-Position: refs/heads/master@{#12060}

webrtc/base/buffer.h

@@ -49,6 +49,7 @@ struct ByteType {
 class Buffer {
  public:
   Buffer();                   // An empty buffer.
+  Buffer(const Buffer& buf);  // Copy size and contents of an existing buffer.
   Buffer(Buffer&& buf);       // Move contents from an existing buffer.
 
   // Construct a buffer with the specified number of uninitialized bytes.
@@ -98,6 +99,12 @@ class Buffer {
     return capacity_;
   }
 
+  Buffer& operator=(const Buffer& buf) {
+    if (&buf != this)
+      SetData(buf.data(), buf.size());
+    return *this;
+  }
+
   Buffer& operator=(Buffer&& buf) {
     RTC_DCHECK(IsConsistent());
     RTC_DCHECK(buf.IsConsistent());
@@ -263,8 +270,6 @@ class Buffer {
   size_t size_;
   size_t capacity_;
   std::unique_ptr<uint8_t[]> data_;
-
-  RTC_DISALLOW_COPY_AND_ASSIGN(Buffer);
 };
 
 }  // namespace rtc