Reland: Wires up WebrtcKeyValueBasedConfig in media engines.

This is a reland of
https://webrtc-review.googlesource.com/c/src/+/174261

Patchset 1 contains the old cl (plus a merge conflict fix).
Later patchets are bufixes: A PeerConnection can be created without a
Call instance (in the case of DataChannel only), so we can't always
use that to fetch the current trials.

Old CL descritpion:

This replaces field_trial:: -based functions from system_wrappers.
Field trials are still used as fallback, but injectable trials are now
possible.

// Since re-land is otherwise unchanged, setting previous reviewers as TBR
TBR=kthelgason@webrtc.org,mbonadei@webrtc.org,stefan@webrtc.org,srte@webrtc.org

Bug: webrtc:11926
Change-Id: I57a9e8c3454f226f77fb93215bcac83da65034b0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/185003
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32163}

media/engine/null_webrtc_video_engine_unittest.cc

@@ -15,6 +15,7 @@
 
 #include "api/task_queue/default_task_queue_factory.h"
 #include "api/task_queue/task_queue_factory.h"
+#include "api/transport/field_trial_based_config.h"
 #include "media/engine/webrtc_voice_engine.h"
 #include "modules/audio_device/include/mock_audio_device.h"
 #include "modules/audio_processing/include/audio_processing.h"
@@ -31,11 +32,12 @@ TEST(NullWebRtcVideoEngineTest, CheckInterface) {
       webrtc::CreateDefaultTaskQueueFactory();
   rtc::scoped_refptr<webrtc::test::MockAudioDeviceModule> adm =
       webrtc::test::MockAudioDeviceModule::CreateNice();
+  webrtc::FieldTrialBasedConfig trials;
   auto audio_engine = std::make_unique<WebRtcVoiceEngine>(
       task_queue_factory.get(), adm,
       webrtc::MockAudioEncoderFactory::CreateUnusedFactory(),
       webrtc::MockAudioDecoderFactory::CreateUnusedFactory(), nullptr,
-      webrtc::AudioProcessingBuilder().Create());
+      webrtc::AudioProcessingBuilder().Create(), trials);
 
   CompositeMediaEngine engine(std::move(audio_engine),
                               std::make_unique<NullWebRtcVideoEngine>());