Revert of base: Make shared memory backed by Mach primitives by default.

base/memory/shared_memory.h

Index: base/memory/shared_memory.h
diff --git a/base/memory/shared_memory.h b/base/memory/shared_memory.h
index 18416c0fd0703b5c0ab56e5174134e4c69308496..5cdcb0d1a31a4f7145958834b4668779e312ea23 100644
--- a/base/memory/shared_memory.h
+++ b/base/memory/shared_memory.h
@@ -140,6 +140,10 @@
   // http://crbug.com/466437.
   bool CreateAndMapAnonymousPosix(size_t size);
   bool CreateAnonymousPosix(size_t size);
+
+  // This method is an analog of CreateAndMapAnonymous that forces the
+  // underlying OS primitive to be a Mach memory object.
+  bool CreateAndMapAnonymousMach(size_t size);
 #endif  // defined(OS_MACOSX) && !defined(OS_IOS)
 
   // Creates an anonymous shared memory segment of size size.