Added explicit EOR to sctp messages and coalesce messages on the receiving side.

TBR=pthatcher@webrtc.org

Bug: webrtc:7774
Change-Id: I41d1cd98d1e7b2ad479177eb2e328a5e2c704824
Reviewed-on: https://webrtc-review.googlesource.com/88900
Commit-Queue: Jeroen de Borst <jeroendb@webrtc.org>
Reviewed-by: Qingsi Wang <qingsi@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24031}

rtc_base/copyonwritebuffer.cc

@@ -22,6 +22,9 @@ CopyOnWriteBuffer::CopyOnWriteBuffer(const CopyOnWriteBuffer& buf)
 CopyOnWriteBuffer::CopyOnWriteBuffer(CopyOnWriteBuffer&& buf)
     : buffer_(std::move(buf.buffer_)) {}
 
+CopyOnWriteBuffer::CopyOnWriteBuffer(const std::string& s)
+    : CopyOnWriteBuffer(s.data(), s.length()) {}
+
 CopyOnWriteBuffer::CopyOnWriteBuffer(size_t size)
     : buffer_(size > 0 ? new RefCountedObject<Buffer>(size) : nullptr) {
   RTC_DCHECK(IsConsistent());