Ensure one does not register same rtp header extension with different id

webrtc/modules/rtp_rtcp/source/rtp_header_extension.cc

Index: webrtc/modules/rtp_rtcp/source/rtp_header_extension.cc
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_header_extension.cc b/webrtc/modules/rtp_rtcp/source/rtp_header_extension.cc
index 27bbda4cc937451a374cd89f3cb48b790716e27e..7fdff3498532ad43aef92592cafc94f463371b6d 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_header_extension.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtp_header_extension.cc
@@ -10,12 +10,15 @@
 
 #include <assert.h>
 
+#include "webrtc/base/checks.h"
 #include "webrtc/common_types.h"
 #include "webrtc/modules/rtp_rtcp/source/rtp_header_extension.h"
 #include "webrtc/modules/rtp_rtcp/source/rtp_utility.h"
 
 namespace webrtc {
 
+constexpr uint8_t RtpHeaderExtensionMap::kInvalidId;
+
 RtpHeaderExtensionMap::RtpHeaderExtensionMap() {
 }
 
@@ -48,6 +51,7 @@ int32_t RtpHeaderExtensionMap::Register(RTPExtensionType type, uint8_t id) {
     // so return success.
     return 0;
   }
+  RTC_DCHECK_EQ(kInvalidId, GetId(type));
   extensionMap_[id] = new HeaderExtension(type);
   return 0;
 }