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/copyonwritebuffer.h

@@ -42,9 +42,7 @@ class CopyOnWriteBuffer {
   template <typename T, typename internal::ByteType<T>::t = 0>
   CopyOnWriteBuffer(const T* data, size_t size, size_t capacity)
       : CopyOnWriteBuffer(size, capacity) {
-    if (buffer_) {
-      std::memcpy(buffer_->data(), data, size);
-    }
+    std::memcpy(buffer_->data(), data, size);
   }
 
   // Construct a buffer from the contents of an array.
@@ -125,24 +123,13 @@ class CopyOnWriteBuffer {
     return !(*this == buf);
   }
 
-  uint8_t& operator[](size_t index) {
-    RTC_DCHECK_LT(index, size());
-    return data()[index];
-  }
-
-  uint8_t operator[](size_t index) const {
-    RTC_DCHECK_LT(index, size());
-    return cdata()[index];
-  }
-
   // Replace the contents of the buffer. Accepts the same types as the
   // constructors.
   template <typename T, typename internal::ByteType<T>::t = 0>
   void SetData(const T* data, size_t size) {
     RTC_DCHECK(IsConsistent());
     if (!buffer_ || !buffer_->HasOneRef()) {
-      buffer_ = size > 0 ? new RefCountedObject<Buffer>(data, size)
-                         : nullptr;
+      buffer_ = new RefCountedObject<Buffer>(data, size, size);
     } else {
       buffer_->SetData(data, size);
     }