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/BUILD.gn b/BUILD.gn
index 95d02f7..9207a1d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -504,6 +504,7 @@
       "rtc_base/synchronization:sequence_checker_unittests",
       "rtc_base/task_utils:to_queued_task_unittests",
       "sdk:sdk_tests",
+      "test:rtp_test_utils",
       "test:test_main",
       "test/network:network_emulation_unittests",
     ]