Support forcing field trial in audioproc_f simulator

This allows the user to run audioproc_f with various field trials set.
The approach is copied from test/test_main_lib.cc.

Tested:
1. Verified bitexactness vs ToT audioproc_f on a large dataset of aecdumps
2. Ran it with flags --aec=1 --force_fieldtrials="WebRTC-Aec3ClampInstQualityToZeroKillSwitch/Enabled/WebRTC-Aec3ClampInstQualityToOneKillSwitch/Enabled/" and verified in GDB that the AEC3 config was changed accordingly.


No-Try: True
Bug: webrtc:5298
Change-Id: I70eec7777f70893b36af33794a5842f67d56af31
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/172623
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30976}

modules/audio_processing/BUILD.gn

@@ -542,6 +542,7 @@ if (rtc_include_tests) {
         "../../rtc_base:task_queue_for_test",
         "../../rtc_base/system:file_wrapper",
         "../../system_wrappers",
+        "../../system_wrappers:field_trial",
         "../../test:test_support",
         "aec_dump",
         "aec_dump:aec_dump_impl",

modules/audio_processing/test/audioproc_float_impl.cc

@@ -27,6 +27,7 @@
 #include "modules/audio_processing/test/wav_based_simulator.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/strings/string_builder.h"
+#include "system_wrappers/include/field_trial.h"
 
 constexpr int kParameterNotSpecifiedValue = -10000;
 
@@ -256,6 +257,13 @@ ABSL_FLAG(bool,
           false,
           "Produce floating point wav output files.");
 
+ABSL_FLAG(std::string,
+          force_fieldtrials,
+          "",
+          "Field trials control experimental feature code which can be forced. "
+          "E.g. running with --force_fieldtrials=WebRTC-FooFeature/Enable/"
+          " will assign the group Enable to field trial WebRTC-FooFeature.");
+
 namespace webrtc {
 namespace test {
 namespace {
@@ -631,6 +639,11 @@ int AudioprocFloatImpl(std::unique_ptr<AudioProcessingBuilder> ap_builder,
     return 1;
   }
 
+  // InitFieldTrialsFromString stores the char*, so the char array must
+  // outlive the application.
+  const std::string field_trials = absl::GetFlag(FLAGS_force_fieldtrials);
+  webrtc::field_trial::InitFieldTrialsFromString(field_trials.c_str());
+
   SimulationSettings settings = CreateSettings();
   if (!input_aecdump.empty()) {
     settings.aec_dump_input_string = input_aecdump;