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/app/webrtc/datachannel.cc b/talk/app/webrtc/datachannel.cc
index 05c934c..1897b73 100644
--- a/talk/app/webrtc/datachannel.cc
+++ b/talk/app/webrtc/datachannel.cc
@@ -330,8 +330,8 @@
   if (was_ever_writable_ && observer_) {
     observer_->OnMessage(*buffer.get());
   } else {
-    if (queued_received_data_.byte_count() + payload.length() >
-            kMaxQueuedReceivedDataBytes) {
+    if (queued_received_data_.byte_count() + payload.size() >
+        kMaxQueuedReceivedDataBytes) {
       LOG(LS_ERROR) << "Queued received data exceeds the max buffer size.";
 
       queued_received_data_.Clear();