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

base/memory/shared_memory_unittest.cc

Index: base/memory/shared_memory_unittest.cc
diff --git a/base/memory/shared_memory_unittest.cc b/base/memory/shared_memory_unittest.cc
index 37b40865cf4bb0ed5bd0f17a359f37841488743a..561c065ba5015b76cab29cd00f2291bb23db03f3 100644
--- a/base/memory/shared_memory_unittest.cc
+++ b/base/memory/shared_memory_unittest.cc
@@ -473,6 +473,10 @@
   SharedMemoryCreateOptions options;
   options.size = kTestSize;
   options.executable = true;
+#if defined(OS_MACOSX) && !defined(OS_IOS)
+  // The Mach functionality is tested in shared_memory_mac_unittest.cc.
+  options.type = SharedMemoryHandle::POSIX;
+#endif
 
   EXPECT_TRUE(shared_memory.Create(options));
   EXPECT_TRUE(shared_memory.Map(shared_memory.requested_size()));