Make the RtpHeaderParserImpl available to tests and tools only.

There are a few reasons for making this test only:
* The code is only used by tests and utilities.
* The pure interface has only a single implementation so an interface isn't really needed.
  (a followup change could remove it altogether)
* The implementation always incorporates locking regardless of how the class gets used.
  See e.g. previous use in the Packet class.
* The implementation is a layer on top of RtpUtility::RtpHeaderParser which is
  sufficient for most production cases.

Change-Id: Ide6d50567cf8ae5127a2eb04cceeb10cf317ec36
Bug: none
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150658
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29010}

modules/rtp_rtcp/source/rtp_rtcp_impl_unittest.cc

@@ -17,7 +17,6 @@
 #include "absl/memory/memory.h"
 #include "api/transport/field_trial_based_config.h"
 #include "api/video_codecs/video_codec.h"
-#include "modules/rtp_rtcp/include/rtp_header_parser.h"
 #include "modules/rtp_rtcp/include/rtp_rtcp_defines.h"
 #include "modules/rtp_rtcp/source/playout_delay_oracle.h"
 #include "modules/rtp_rtcp/source/rtcp_packet.h"
@@ -28,6 +27,7 @@
 #include "test/gmock.h"
 #include "test/gtest.h"
 #include "test/rtcp_packet_parser.h"
+#include "test/rtp_header_parser.h"
 
 using ::testing::_;
 using ::testing::ElementsAre;
@@ -72,7 +72,7 @@ class SendTransport : public Transport {
                size_t len,
                const PacketOptions& options) override {
     RTPHeader header;
-    std::unique_ptr<RtpHeaderParser> parser(RtpHeaderParser::Create());
+    std::unique_ptr<RtpHeaderParser> parser(RtpHeaderParser::CreateForTest());
     EXPECT_TRUE(parser->Parse(static_cast<const uint8_t*>(data), len, &header));
     ++rtp_packets_sent_;
     last_rtp_header_ = header;