Rename RTP-extension constants.

BUG=
R=mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/3969004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@5137 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/video/video_send_stream.cc b/video/video_send_stream.cc
index db1cb02..cc723b6 100644
--- a/video/video_send_stream.cc
+++ b/video/video_send_stream.cc
@@ -123,10 +123,10 @@
   for (size_t i = 0; i < config_.rtp.extensions.size(); ++i) {
     const std::string& extension = config_.rtp.extensions[i].name;
     int id = config_.rtp.extensions[i].id;
-    if (extension == "toffset") {
+    if (extension == RtpExtension::kTOffset) {
       if (rtp_rtcp_->SetSendTimestampOffsetStatus(channel_, true, id) != 0)
         abort();
-    } else if (extension == "abs-send-time") {
+    } else if (extension == RtpExtension::kAbsSendTime) {
       if (rtp_rtcp_->SetSendAbsoluteSendTimeStatus(channel_, true, id) != 0)
         abort();
     } else {