Replace Atomic32 with webrtc/base/atomicops.h.

webrtc/system_wrappers/BUILD.gn

Index: webrtc/system_wrappers/BUILD.gn
diff --git a/webrtc/system_wrappers/BUILD.gn b/webrtc/system_wrappers/BUILD.gn
index 0657e20426a330eaba2fdaedac2099076ef632ad..ac112137d8f931d78ade263899786153c59df35a 100644
--- a/webrtc/system_wrappers/BUILD.gn
+++ b/webrtc/system_wrappers/BUILD.gn
@@ -13,7 +13,6 @@ static_library("system_wrappers") {
   sources = [
     "interface/aligned_array.h",
     "interface/aligned_malloc.h",
-    "interface/atomic32.h",
     "interface/clock.h",
     "interface/condition_variable_wrapper.h",
     "interface/cpu_features_wrapper.h",
@@ -45,8 +44,6 @@ static_library("system_wrappers") {
     "interface/trace_event.h",
     "interface/utf_util_win.h",
     "source/aligned_malloc.cc",
-    "source/atomic32_mac.cc",
-    "source/atomic32_win.cc",
     "source/clock.cc",
     "source/condition_variable.cc",
     "source/condition_variable_event_win.cc",
@@ -145,10 +142,6 @@ static_library("system_wrappers") {
     libs += [ "rt" ]
   }
 
-  if (!is_mac && !is_ios) {
-    sources += [ "source/atomic32_posix.cc" ]
-  }
-
   if (is_ios || is_mac) {
     defines += [
       "WEBRTC_THREAD_RR",
@@ -156,10 +149,6 @@ static_library("system_wrappers") {
     ]
   }
 
-  if (is_ios) {
-    sources += [ "source/atomic32_mac.cc" ]
-  }
-
   if (is_win) {
     libs += [ "winmm.lib" ]