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}
diff --git a/sdk/BUILD.gn b/sdk/BUILD.gn
index a50b93d..9a04322 100644
--- a/sdk/BUILD.gn
+++ b/sdk/BUILD.gn
@@ -213,7 +213,6 @@
           "../rtc_base:checks",
           "../rtc_base:rtc_base_approved",
           "../system_wrappers",
-          "../system_wrappers:metrics_api",
         ]
 
         if (is_clang) {
@@ -260,7 +259,7 @@
           "../rtc_base:checks",
           "../rtc_base:rtc_base",
           "../rtc_base/system:fallthrough",
-          "../system_wrappers:metrics_api",
+          "../system_wrappers:metrics",
         ]
 
         if (is_clang) {
@@ -564,7 +563,6 @@
         "../modules/video_coding:video_codec_interface",
         "../rtc_base:checks",
         "../rtc_base:rtc_base_approved",
-        "../system_wrappers:field_trial_api",
       ]
     }
 
@@ -614,7 +612,6 @@
         ":base_objc",
         ":wrapped_native_codec_objc",
         "../modules/video_coding:webrtc_vp8",
-        "../system_wrappers:metrics_default",
       ]
     }
 
@@ -633,7 +630,6 @@
         ":wrapped_native_codec_objc",
         "../media:rtc_media_base",
         "../modules/video_coding:webrtc_vp9",
-        "../system_wrappers:metrics_default",
       ]
     }
 
@@ -921,6 +917,8 @@
         "../pc:peerconnection",
         "../rtc_base:checks",
         "../rtc_base:rtc_base",
+        "../system_wrappers:field_trial",
+        "../system_wrappers:metrics",
         "//third_party/abseil-cpp/absl/memory",
       ]
 
@@ -1310,8 +1308,6 @@
           ":videocodec_objc",
           ":videotoolbox_objc",
           "../rtc_base:rtc_base_approved",
-          "../system_wrappers:field_trial_default",
-          "../system_wrappers:metrics_default",
           "../system_wrappers:runtime_enabled_features_default",
         ]
         if (rtc_use_metal_rendering) {
@@ -1452,8 +1448,6 @@
           ":videocodec_objc",
           ":videotoolbox_objc",
           "../rtc_base:rtc_base_approved",
-          "../system_wrappers:field_trial_default",
-          "../system_wrappers:metrics_default",
           "../system_wrappers:runtime_enabled_features_default",
         ]
         if (rtc_use_metal_rendering) {