Mark rtc_base/memory:fifo_buffer as testonly

Bug: webrtc:6424
Change-Id: Ifae66027f2cd308650b07dd4b02bcb1d75a69111
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/213144
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33581}

p2p/BUILD.gn

@@ -118,7 +118,6 @@ rtc_library("rtc_p2p") {
     # Needed by pseudo_tcp, which should move to a separate target.
     "../rtc_base:safe_minmax",
     "../rtc_base:weak_ptr",
-    "../rtc_base/memory:fifo_buffer",
     "../rtc_base/network:sent_packet",
     "../rtc_base/synchronization:mutex",
     "../rtc_base/system:rtc_export",

rtc_base/memory/BUILD.gn

@@ -21,14 +21,12 @@ rtc_library("aligned_malloc") {
 }
 
 # Test only utility.
-# TODO: Tag with `testonly = true` once all depending targets are correctly
-# tagged.
 rtc_library("fifo_buffer") {
+  testonly = true
   visibility = [
     ":unittests",
     "..:rtc_base_tests_utils",
     "..:rtc_base_unittests",
-    "../../p2p:rtc_p2p",  # This needs to be fixed.
   ]
   sources = [
     "fifo_buffer.cc",