Split parts of //chrome/utility's BUILD.gn into //chrome/utility/safe_browsing/mac.

chrome/utility/safe_browsing/mac/crdmg.cc

Index: chrome/utility/safe_browsing/mac/crdmg.cc
diff --git a/chrome/utility/safe_browsing/mac/crdmg.cc b/chrome/utility/safe_browsing/mac/crdmg.cc
index 18a5ea1535de467b5b70d4971d046641e9ebcc83..8dc0bc9d3cf440aa3348603fa3f090acb6cb056b 100644
--- a/chrome/utility/safe_browsing/mac/crdmg.cc
+++ b/chrome/utility/safe_browsing/mac/crdmg.cc
@@ -25,6 +25,14 @@
 #include "chrome/utility/safe_browsing/mac/read_stream.h"
 #include "chrome/utility/safe_browsing/mac/udif.h"
 
+// This executable only works on 10.10+, so unconditionally use these functions
+// to make sandboxing easier.
+extern "C" {
+int mkdirat(int, const char *, mode_t);
+int openat(int, const char *, int, ...);
+int unlinkat(int, const char *, int);
+}
+
 namespace {
 
 // SafeDMG (crdmg) is a utility that can perform a list or extract operation