Migrate WebRTC test infra to ABSL_FLAG.

This is the last CL required to migrate WebRTC to ABSL_FLAG, rtc::Flag
will be removed soon after this one lands.

Bug: webrtc:10616
Change-Id: I2807cec39e28a2737d2c49e2dc23f2a6f98d08f0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/145727
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28606}

api/BUILD.gn

diff --git a/api/BUILD.gn b/api/BUILD.gn
index 645daee..ca2b5cb 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -573,7 +573,8 @@
         ":neteq_simulator_api",
         "../modules/audio_coding:neteq_test_factory",
         "../rtc_base:checks",
-        "../rtc_base:rtc_base_approved",
+        "//third_party/abseil-cpp/absl/flags:flag",
+        "//third_party/abseil-cpp/absl/flags:parse",
         "//third_party/abseil-cpp/absl/memory",
         "//third_party/abseil-cpp/absl/strings",
       ]

api/test/neteq_simulator_factory.cc

diff --git a/api/test/neteq_simulator_factory.cc b/api/test/neteq_simulator_factory.cc
index 64e9c5b..8bb0625 100644
--- a/api/test/neteq_simulator_factory.cc
+++ b/api/test/neteq_simulator_factory.cc
@@ -11,21 +11,25 @@
 #include "api/test/neteq_simulator_factory.h"
 
 #include <string>
+#include <vector>
 
+#include "absl/flags/flag.h"
+#include "absl/flags/parse.h"
 #include "absl/memory/memory.h"
 #include "modules/audio_coding/neteq/tools/neteq_test_factory.h"
 #include "rtc_base/checks.h"
-#include "rtc_base/flags.h"
 
 namespace {
 
-WEBRTC_DEFINE_string(replacement_audio_file,
-                     "",
-                     "A PCM file that will be used to populate dummy"
-                     " RTP packets");
-WEBRTC_DEFINE_int(max_nr_packets_in_buffer,
-                  50,
-                  "Maximum allowed number of packets in the buffer");
+ABSL_FLAG(std::string,
+          replacement_audio_file,
+          "",
+          "A PCM file that will be used to populate dummy"
+          " RTP packets");
+ABSL_FLAG(int,
+          max_nr_packets_in_buffer,
+          50,
+          "Maximum allowed number of packets in the buffer");
 
 }  // namespace
 
@@ -40,17 +44,17 @@
 std::unique_ptr<NetEqSimulator> NetEqSimulatorFactory::CreateSimulator(
     int argc,
     char* argv[]) {
-  RTC_CHECK(!rtc::FlagList::SetFlagsFromCommandLine(&argc, argv, true))
-      << "Error while parsing command-line flags";
-  RTC_CHECK_EQ(argc, 3) << "Wrong number of input arguments. Expected 3, got "
-                        << argc;
+  std::vector<char*> args = absl::ParseCommandLine(argc, argv);
+  RTC_CHECK_EQ(args.size(), 3)
+      << "Wrong number of input arguments. Expected 3, got " << args.size();
   // TODO(ivoc) Stop (ab)using command-line flags in this function.
-  const std::string output_audio_filename(argv[2]);
+  const std::string output_audio_filename(args[2]);
   NetEqTestFactory::Config config;
-  config.replacement_audio_file = FLAG_replacement_audio_file;
-  config.max_nr_packets_in_buffer = FLAG_max_nr_packets_in_buffer;
+  config.replacement_audio_file = absl::GetFlag(FLAGS_replacement_audio_file);
+  config.max_nr_packets_in_buffer =
+      absl::GetFlag(FLAGS_max_nr_packets_in_buffer);
   config.output_audio_filename = output_audio_filename;
-  return factory_->InitializeTestFromFile(argv[1], config);
+  return factory_->InitializeTestFromFile(args[1], config);
 }
 
 std::unique_ptr<NetEqSimulator> NetEqSimulatorFactory::CreateSimulatorFromFile(