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}
diff --git a/webrtc/api/datachannelinterface.h b/webrtc/api/datachannelinterface.h
index 53c11d4..3d6f711 100644
--- a/webrtc/api/datachannelinterface.h
+++ b/webrtc/api/datachannelinterface.h
@@ -17,8 +17,8 @@
 #include <string>
 
 #include "webrtc/base/basictypes.h"
+#include "webrtc/base/buffer.h"
 #include "webrtc/base/checks.h"
-#include "webrtc/base/copyonwritebuffer.h"
 #include "webrtc/base/refcount.h"
 
 
@@ -50,7 +50,7 @@
 };
 
 struct DataBuffer {
-  DataBuffer(const rtc::CopyOnWriteBuffer& data, bool binary)
+  DataBuffer(const rtc::Buffer& data, bool binary)
       : data(data),
         binary(binary) {
   }
@@ -61,7 +61,7 @@
   }
   size_t size() const { return data.size(); }
 
-  rtc::CopyOnWriteBuffer data;
+  rtc::Buffer data;
   // Indicates if the received data contains UTF-8 or binary data.
   // Note that the upper layers are left to verify the UTF-8 encoding.
   // TODO(jiayl): prefer to use an enum instead of a bool.