Remove rtc::Buffer::length(), since no one uses it anymore

Chromium now uses size() instead, just like WebRTC.

This CL also fixes a new length() call that had crept in.

R=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44119004

Cr-Commit-Position: refs/heads/master@{#9024}
diff --git a/webrtc/base/buffer.h b/webrtc/base/buffer.h
index 07345a9..c7fb959 100644
--- a/webrtc/base/buffer.h
+++ b/webrtc/base/buffer.h
@@ -34,10 +34,6 @@
   size_t size() const { return size_; }
   size_t capacity() const { return capacity_; }
 
-  // For backwards compatibility. TODO(kwiberg): Remove once Chromium doesn't
-  // need it anymore.
-  size_t length() const { return size(); }
-
   Buffer& operator=(const Buffer& buf) {
     if (&buf != this) {
       Construct(buf.data(), buf.size(), buf.size());
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_sender_unittest.cc b/webrtc/modules/rtp_rtcp/source/rtp_sender_unittest.cc
index d04c636..8a85fbe 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_sender_unittest.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtp_sender_unittest.cc
@@ -1361,7 +1361,7 @@
   // Verify that this packet does have CVO byte.
   VerifyCVOPacket(
       reinterpret_cast<uint8_t*>(transport_.sent_packets_[0]->data()),
-      transport_.sent_packets_[0]->length(), true, &map, kSeqNum, hdr.rotation);
+      transport_.sent_packets_[0]->size(), true, &map, kSeqNum, hdr.rotation);
 
   // Verify that this packet does have CVO byte.
   VerifyCVOPacket(