Move implmentation specific constants out of rtp_header_extension.h

webrtc/modules/rtp_rtcp/source/rtp_header_extension_unittest.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 69 matching lines @@
   EXPECT_TRUE(map.Register<TransmissionOffset>(3));
 
   EXPECT_FALSE(map.Register<AudioLevel>(3));
   EXPECT_TRUE(map.Register<AudioLevel>(4));
 }
 
 TEST(RtpHeaderExtensionTest, GetTotalLength) {
   RtpHeaderExtensionMap map;
   EXPECT_EQ(0u, map.GetTotalLengthInBytes());
   EXPECT_TRUE(map.Register<TransmissionOffset>(3));
+  static constexpr size_t kRtpOneByteHeaderLength = 4;
   EXPECT_EQ(kRtpOneByteHeaderLength + (TransmissionOffset::kValueSizeBytes + 1),
             map.GetTotalLengthInBytes());
 }
 
 TEST(RtpHeaderExtensionTest, GetType) {
   RtpHeaderExtensionMap map;
   EXPECT_EQ(RtpHeaderExtensionMap::kInvalidType, map.GetType(3));
   EXPECT_TRUE(map.Register<TransmissionOffset>(3));
 
   EXPECT_EQ(TransmissionOffset::kId, map.GetType(3));
 }
 
 TEST(RtpHeaderExtensionTest, GetId) {
   RtpHeaderExtensionMap map;
   EXPECT_EQ(RtpHeaderExtensionMap::kInvalidId,
             map.GetId(TransmissionOffset::kId));
   EXPECT_TRUE(map.Register<TransmissionOffset>(3));
 
   EXPECT_EQ(3, map.GetId(TransmissionOffset::kId));
 }
 
 }  // namespace webrtc