rtc::Buffer: Rename length to size, for conformance with the STL

And add a constructor for creating an uninitialized Buffer of a
specified size.

(I intend to follow up with more Buffer changes, but since it's rather
widely used, the rename is quite noisy and works better as a separate
CL.)

R=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/48579004

Cr-Commit-Position: refs/heads/master@{#8841}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8841 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/talk/media/sctp/sctpdataengine_unittest.cc b/talk/media/sctp/sctpdataengine_unittest.cc
index 3a050cc..5b4c09e 100644
--- a/talk/media/sctp/sctpdataengine_unittest.cc
+++ b/talk/media/sctp/sctpdataengine_unittest.cc
@@ -74,7 +74,7 @@
 
     // TODO(ldixon): Can/should we use Buffer.TransferTo here?
     // Note: this assignment does a deep copy of data from packet.
-    rtc::Buffer* buffer = new rtc::Buffer(packet->data(), packet->length());
+    rtc::Buffer* buffer = new rtc::Buffer(packet->data(), packet->size());
     thread_->Post(this, MSG_PACKET, rtc::WrapMessageData(buffer));
     LOG(LS_VERBOSE) << "SctpFakeNetworkInterface::SendPacket, Posted message.";
     return true;