Move implmentation specific constants out of rtp_header_extension.h

webrtc/modules/rtp_rtcp/source/rtp_header_extension.cc

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 78 matching lines @@
 bool RtpHeaderExtensionMap::RegisterByUri(uint8_t id, const std::string& uri) {
   for (const ExtensionInfo& extension : kExtensions)
     if (uri == extension.uri)
       return Register(id, extension.type, extension.value_size, extension.uri);
   LOG(LS_WARNING) << "Unknown extension uri:'" << uri
                   << "', id: " << static_cast<int>(id) << '.';
   return false;
 }
 
 size_t RtpHeaderExtensionMap::GetTotalLengthInBytes() const {
+  static constexpr size_t kRtpOneByteHeaderLength = 4;
   if (total_values_size_bytes_ == 0)
     return 0;
   return Word32Align(kRtpOneByteHeaderLength + total_values_size_bytes_);
 }
 
 int32_t RtpHeaderExtensionMap::Deregister(RTPExtensionType type) {
   if (IsRegistered(type)) {
     uint8_t id = GetId(type);
     total_values_size_bytes_ -= (ValueSize(type) + 1);
     types_[id] = kInvalidType;
@@ skipping 32 matching lines @@
   }
   RTC_DCHECK(!IsRegistered(type));
 
   types_[id] = type;
   ids_[type] = id;
   total_values_size_bytes_ += (value_size + 1);
   return true;
 }
 
 }  // namespace webrtc