Decoupling rtc_base from apple specific code [without cyclic deps]

webrtc/rtc_base/BUILD.gn

 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("//build/config/crypto.gni")
 import("//build/config/ui.gni")
@@ skipping 380 matching lines @@
     ]
   } else {
     include_dirs = [ "$rtc_jsoncpp_root" ]
 
     # When defined changes the include path for json.h to where it is
     # expected to be when building json outside of the standalone build.
     defines += [ "WEBRTC_EXTERNAL_JSON" ]
   }
 }
 
+# TODO(mbonadei): make this target available only if is_ios or is_mac
+# are true.
+rtc_static_library("rtc_base_apple") {
+  sources = [
+    "applefilesystem.mm",
+    "thread_darwin.mm",
+  ]
+  deps = [
+    ":rtc_base",
+  ]
+}
+
 rtc_static_library("rtc_base") {
   cflags = []
   cflags_cc = []
   libs = []
   defines = []
   deps = [
     "..:webrtc_common",
   ]
   public_deps = [
     ":rtc_base_approved",
   ]
   public_configs = []
 
   all_dependent_configs = [ ":rtc_base_all_dependent_config" ]
 
   sources = [
-    "applefilesystem.mm",
     "asyncinvoker-inl.h",
     "asyncinvoker.cc",
     "asyncinvoker.h",
     "asyncpacketsocket.cc",
     "asyncpacketsocket.h",
     "asyncresolverinterface.cc",
     "asyncresolverinterface.h",
     "asyncsocket.cc",
     "asyncsocket.h",
     "asynctcpsocket.cc",
@@ skipping 132 matching lines @@
       "ifaddrs-android.h",
     ]
 
     libs += [
       "log",
       "GLESv2",
     ]
   }
 
   if (is_ios || is_mac) {
-    sources += [
-      "macifaddrs_converter.cc",
-      "thread_darwin.mm",
-    ]
+    sources += [ "macifaddrs_converter.cc" ]
   }
 
   if (use_x11) {
     libs += [
       "dl",
       "rt",
       "Xext",
       "X11",
       "Xcomposite",
       "Xrender",
@@ skipping 447 matching lines @@
 if (is_android) {
   android_library("base_java") {
     java_files = [
       "java/src/org/webrtc/ContextUtils.java",
       "java/src/org/webrtc/Logging.java",
       "java/src/org/webrtc/Size.java",
       "java/src/org/webrtc/ThreadUtils.java",
     ]
   }
 }