Remove deprecated field_trial_default and metrics_default.

This CL removes some deprecated build targets (and their headers)
from system_wrappers:
- field_trial_api
- field_trial_default
- metrics_api
- metrics_default

It also refreshes all the dependencies on field_trial.h and metrics.h.

A nice side effect is that it is finally possible to remove 'nogncheck'
from the following files (when it was used with field_trial_default
and metrics_default):
- sdk/objc/api/peerconnection/RTCMetricsSampleInfo+Private.h
- sdk/android/src/jni/pc/peerconnectionfactory.cc
- sdk/objc/api/peerconnection/RTCFieldTrials.mm

Bug: webrtc:9631
Change-Id: Ib621f41ef8ad0aba4fe1c1d7e749c044afc956c3
No-Try: True
Reviewed-on: https://webrtc-review.googlesource.com/100524
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24878}

sdk/objc/api/peerconnection/RTCFieldTrials.h

@@ -38,9 +38,8 @@ RTC_EXTERN NSString * const kRTCFieldTrialTrendlineFilterKey;
 RTC_EXTERN NSString *RTCFieldTrialTrendlineFilterValue(
     size_t windowSize, double smoothingCoeff, double thresholdGain);
 
-/** Initialize field trials using a dictionary mapping field trial keys to their values. See above
- *  for valid keys and values.
- *  Must be called before any other call into WebRTC. See:
- *  webrtc/system_wrappers/include/field_trial_default.h
+/** Initialize field trials using a dictionary mapping field trial keys to their
+ * values. See above for valid keys and values. Must be called before any other
+ * call into WebRTC. See: webrtc/system_wrappers/include/field_trial.h
  */
 RTC_EXTERN void RTCInitFieldTrialDictionary(NSDictionary<NSString *, NSString *> *fieldTrials);

sdk/objc/api/peerconnection/RTCFieldTrials.mm

@@ -14,10 +14,7 @@
 
 #import "base/RTCLogging.h"
 
-// Adding 'nogncheck' to disable the gn include headers check.
-// We don't want to depend on 'system_wrappers:field_trial_default' because
-// clients should be able to provide their own implementation.
-#include "system_wrappers/include/field_trial_default.h"  // nogncheck
+#include "system_wrappers/include/field_trial.h"
 
 NSString * const kRTCFieldTrialAudioSendSideBweKey = @"WebRTC-Audio-SendSideBwe";
 NSString * const kRTCFieldTrialAudioSendSideBweForVideoKey = @"WebRTC-Audio-SendSideBwe-For-Video";

sdk/objc/api/peerconnection/RTCMetricsSampleInfo+Private.h

@@ -10,10 +10,7 @@
 
 #import "RTCMetricsSampleInfo.h"
 
-// Adding 'nogncheck' to disable the gn include headers check.
-// We don't want to depend on 'system_wrappers:metrics_default' because
-// clients should be able to provide their own implementation.
-#include "system_wrappers/include/metrics_default.h"  // nogncheck
+#include "system_wrappers/include/metrics.h"
 
 NS_ASSUME_NONNULL_BEGIN