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}

webrtc/modules/rtp_rtcp/source/rtp_sender_unittest.cc

@@ -1361,7 +1361,7 @@ TEST_F(RtpSenderVideoTest, SendVideoWithCVO) {
   // 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(