Add Darwin thread.h implementation.

webrtc/base/BUILD.gn

Index: webrtc/base/BUILD.gn
diff --git a/webrtc/base/BUILD.gn b/webrtc/base/BUILD.gn
index 7514b07c8df48ea08e78404b6c35d40e94f6327d..4f563d11920c48b75a54a5fb89ba21f2d149e7dc 100644
--- a/webrtc/base/BUILD.gn
+++ b/webrtc/base/BUILD.gn
@@ -544,11 +544,8 @@ rtc_static_library("rtc_base") {
 
   if (is_ios || is_mac) {
     sources += [
-      "maccocoathreadhelper.h",
-      "maccocoathreadhelper.mm",
+      "cocoathread.mm",

[tommi, 2017/03/28 13:59:01]

I wonder if we should call this file thread_cocoa.mm to keep in line with the
thread_<platform>.<ext> convention. I often use it when working in these files
and think that if someone else is doing the same, it can be good for this file
to pop up alongside the others.

[kthelgason, 2017/03/28 14:29:58]

Done.

       "macifaddrs_converter.cc",
-      "scoped_autorelease_pool.h",
-      "scoped_autorelease_pool.mm",
     ]
   }