Remove OS(IOS) and OS(MAC_OS_X) guards.

Source/wtf/ThreadingPthreads.cpp

Index: Source/wtf/ThreadingPthreads.cpp
diff --git a/Source/wtf/ThreadingPthreads.cpp b/Source/wtf/ThreadingPthreads.cpp
index 21adbda1153c37e3a0d0c9a11b6bad9fe3190334..0383074d70c58a6a451d05e88e2917028fc90d48 100644
--- a/Source/wtf/ThreadingPthreads.cpp
+++ b/Source/wtf/ThreadingPthreads.cpp
@@ -55,7 +55,7 @@
 #include <sys/time.h>
 #endif
 
-#if OS(MAC_OS_X) && __MAC_OS_X_VERSION_MIN_REQUIRED >= 1060
+#if OS(DARWIN) && __MAC_OS_X_VERSION_MIN_REQUIRED >= 1060
 #include <objc/objc-auto.h>
 #endif
 
@@ -205,7 +205,7 @@ void initializeCurrentThreadInternal(const char* threadName)
     UNUSED_PARAM(threadName);
 #endif
 
-#if OS(MAC_OS_X) && __MAC_OS_X_VERSION_MIN_REQUIRED >= 1060
+#if OS(DARWIN) && __MAC_OS_X_VERSION_MIN_REQUIRED >= 1060
     // All threads that potentially use APIs above the BSD layer must be registered with the Objective-C
     // garbage collector in case API implementations use garbage-collected memory.
     objc_registerThreadWithCollector();