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}
diff --git a/test/rtp_rtcp_observer.h b/test/rtp_rtcp_observer.h
index 830c2f1..5763039 100644
--- a/test/rtp_rtcp_observer.h
+++ b/test/rtp_rtcp_observer.h
@@ -18,12 +18,12 @@
 #include "api/test/simulated_network.h"
 #include "call/simulated_packet_receiver.h"
 #include "call/video_send_stream.h"
-#include "modules/rtp_rtcp/include/rtp_header_parser.h"
 #include "rtc_base/critical_section.h"
 #include "rtc_base/event.h"
 #include "system_wrappers/include/field_trial.h"
 #include "test/direct_transport.h"
 #include "test/gtest.h"
+#include "test/rtp_header_parser.h"
 
 namespace {
 const int kShortTimeoutMs = 500;
@@ -71,7 +71,8 @@
  protected:
   RtpRtcpObserver() : RtpRtcpObserver(0) {}
   explicit RtpRtcpObserver(int event_timeout_ms)
-      : parser_(RtpHeaderParser::Create()), timeout_ms_(event_timeout_ms) {}
+      : parser_(RtpHeaderParser::CreateForTest()),
+        timeout_ms_(event_timeout_ms) {}
 
   rtc::Event observation_complete_;
   const std::unique_ptr<RtpHeaderParser> parser_;