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}

test/fuzzers/rtp_header_parser_fuzzer.cc

@@ -15,7 +15,7 @@
 #include <memory>
 #include <string>
 
-#include "modules/rtp_rtcp/include/rtp_header_parser.h"
+#include "test/rtp_header_parser.h"
 
 namespace webrtc {
 
@@ -24,7 +24,8 @@ void FuzzOneInput(const uint8_t* data, size_t size) {
   RtpHeaderParser::GetSsrc(data, size);
   RTPHeader rtp_header;
 
-  std::unique_ptr<RtpHeaderParser> rtp_header_parser(RtpHeaderParser::Create());
+  std::unique_ptr<RtpHeaderParser> rtp_header_parser(
+      RtpHeaderParser::CreateForTest());
 
   rtp_header_parser->Parse(data, size, &rtp_header);
   for (int i = 1; i < kRtpExtensionNumberOfExtensions; ++i) {