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/objc/RTCDataChannel.mm b/webrtc/api/objc/RTCDataChannel.mm
index 9859905..5778cb5 100644
--- a/webrtc/api/objc/RTCDataChannel.mm
+++ b/webrtc/api/objc/RTCDataChannel.mm
@@ -53,8 +53,8 @@
 - (instancetype)initWithData:(NSData *)data isBinary:(BOOL)isBinary {
   NSParameterAssert(data);
   if (self = [super init]) {
-    rtc::CopyOnWriteBuffer buffer(
-        reinterpret_cast<const uint8_t*>(data.bytes), data.length);
+    rtc::Buffer buffer(reinterpret_cast<const uint8_t*>(data.bytes),
+                       data.length);
     _dataBuffer.reset(new webrtc::DataBuffer(buffer, isBinary));
   }
   return self;