Reland of Decoupling rtc_base from Obj-C code

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 03e8ea608a78c6420eca399840014234c45461e5..743229302b4d7694752ac2f43c1f07f275e668fc 100644
--- a/webrtc/rtc_base/BUILD.gn
+++ b/webrtc/rtc_base/BUILD.gn
@@ -429,7 +429,28 @@
   }
 }
 
-rtc_static_library("rtc_base") {
+rtc_source_set("rtc_base") {
+  public_deps = [
+    ":rtc_base_generic",
+  ]
+  if (is_ios || is_mac) {
+    public_deps += [ ":rtc_base_objc" ]
+  }
+}
+
+if (is_ios || is_mac) {
+  rtc_source_set("rtc_base_objc") {
+    sources = [
+      "thread_darwin.mm",
+    ]
+    deps = [
+      ":rtc_base_generic",
+    ]
+    visibility = [ ":rtc_base" ]
+  }
+}
+
+rtc_source_set("rtc_base_generic") {
   cflags = []
   cflags_cc = []
   libs = []
@@ -538,6 +559,11 @@
     "thread.h",
   ]
 
+  visibility = [
+    ":rtc_base",
+    ":rtc_base_objc",
+  ]
+
   # TODO(henrike): issue 3307, make rtc_base build with the Chromium default
   # compiler settings.
   suppressed_configs += [ "//build/config/compiler:chromium_code" ]
@@ -597,10 +623,7 @@
   }
 
   if (is_ios || is_mac) {
-    sources += [
-      "macifaddrs_converter.cc",
-      "thread_darwin.mm",
-    ]
+    sources += [ "macifaddrs_converter.cc" ]
   }
 
   if (use_x11) {