Decoupling rtc_base from apple specific code [without cyclic deps]

webrtc/rtc_base/BUILD.gn

Index: webrtc/rtc_base/BUILD.gn
diff --git a/webrtc/rtc_base/BUILD.gn b/webrtc/rtc_base/BUILD.gn
index 851973b15a07cb06ac9a8f6bb7604650c8be81c0..b9567f9c13a32657b0c5a5d3925ce8d56ae9693e 100644
--- a/webrtc/rtc_base/BUILD.gn
+++ b/webrtc/rtc_base/BUILD.gn
@@ -398,6 +398,18 @@ rtc_source_set("rtc_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 = []
@@ -414,7 +426,6 @@ rtc_static_library("rtc_base") {
   all_dependent_configs = [ ":rtc_base_all_dependent_config" ]
 
   sources = [
-    "applefilesystem.mm",
     "asyncinvoker-inl.h",
     "asyncinvoker.cc",
     "asyncinvoker.h",
@@ -567,10 +578,7 @@ rtc_static_library("rtc_base") {
   }
 
   if (is_ios || is_mac) {
-    sources += [
-      "macifaddrs_converter.cc",
-      "thread_darwin.mm",
-    ]
+    sources += [ "macifaddrs_converter.cc" ]
   }
 
   if (use_x11) {