Add Darwin thread.h implementation.

webrtc/base/thread.h

Index: webrtc/base/thread.h
diff --git a/webrtc/base/thread.h b/webrtc/base/thread.h
index 5751df385cc5d1acd43f8b153d3af0720ceb7a6f..c83ea4851348bcc7bfa786abe5348440e8dc1c90 100644
--- a/webrtc/base/thread.h
+++ b/webrtc/base/thread.h
@@ -91,6 +91,12 @@ class Runnable {
   RTC_DISALLOW_COPY_AND_ASSIGN(Runnable);
 };
 
+struct ThreadInit {

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

can we move this declaration into the source file that needs it? Seems like an
implementation detail.

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

I moved it here as it's used in both thread.cc and thread_darwin.mm, I didn't
want to duplicate it in both places. I agree that it's an implementation detail,
so if you want to move it I will.

[tommi, 2017/03/28 14:38:56]

ah, can it be a private struct in Thread?

+  Thread* thread;
+  Runnable* runnable;
+};
+
+
 // WARNING! SUBCLASSES MUST CALL Stop() IN THEIR DESTRUCTORS!  See ~Thread().
 
 class LOCKABLE Thread : public MessageQueue {