Fix iOS GN build and cleanup system_wrappers

Compile fixes for GN on iOS that finally gets our bots green.

Changes to system_wrappers:
* Updated to only use inclusive sources for maintainability
* Add a few missing GN headers.
* Cleanup GYP hack for atomic32_mac.cc
* Renamed changes sources to avoid problems with GYP/GN file
   suffix rules:
  - atomic32_mac.cc -> atomic32_darwin.cc
  - atomic32_posix.cc -> atomic32_non_darwin_unix.cc
See https://code.google.com/p/chromium/codesearch#chromium/src/build/config/BUILDCONFIG.gn&l=325
for details on which extensions can/cannot be used.

BUG=webrtc:5586
NOTRY=True

Review-Url: https://codereview.webrtc.org/1999723002
Cr-Commit-Position: refs/heads/master@{#12897}
diff --git a/webrtc/system_wrappers/BUILD.gn b/webrtc/system_wrappers/BUILD.gn
index da785e2..435418f 100644
--- a/webrtc/system_wrappers/BUILD.gn
+++ b/webrtc/system_wrappers/BUILD.gn
@@ -27,6 +27,7 @@
     "include/fix_interlocked_exchange_pointer_win.h",
     "include/logging.h",
     "include/metrics.h",
+    "include/ntp_time.h",
     "include/rtp_to_ntp.h",
     "include/rw_lock_wrapper.h",
     "include/sleep.h",
@@ -38,7 +39,6 @@
     "include/trace.h",
     "include/utf_util_win.h",
     "source/aligned_malloc.cc",
-    "source/atomic32_mac.cc",
     "source/atomic32_win.cc",
     "source/clock.cc",
     "source/condition_variable_event_win.cc",
@@ -112,16 +112,13 @@
     libs += [ "rt" ]
   }
 
-  if (!is_mac && !is_ios) {
-    sources += [ "source/atomic32_posix.cc" ]
+  if (is_linux || is_android) {
+    sources += [ "source/atomic32_non_darwin_unix.cc" ]
   }
 
   if (is_ios || is_mac) {
     defines += [ "WEBRTC_THREAD_RR" ]
-  }
-
-  if (is_ios) {
-    sources += [ "source/atomic32_mac.cc" ]
+    sources += [ "source/atomic32_darwin.cc" ]
   }
 
   if (is_win) {