Reland 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

Review URL: https://codereview.webrtc.org/1823503002

Cr-Commit-Position: refs/heads/master@{#12062}
diff --git a/webrtc/api/peerconnectioninterface_unittest.cc b/webrtc/api/peerconnectioninterface_unittest.cc
index 6bff11a..d4678ba 100644
--- a/webrtc/api/peerconnectioninterface_unittest.cc
+++ b/webrtc/api/peerconnectioninterface_unittest.cc
@@ -1400,7 +1400,7 @@
   EXPECT_EQ(DataChannelInterface::kOpen, data1->state());
   EXPECT_EQ(DataChannelInterface::kOpen, data2->state());
 
-  rtc::Buffer buffer("test", 4);
+  rtc::CopyOnWriteBuffer buffer("test", 4);
   EXPECT_FALSE(data1->Send(DataBuffer(buffer, true)));
 }