[cryptohome] make guest-mode homedir mount with -rwx------

platform.cc

Index: platform.cc
diff --git a/platform.cc b/platform.cc
index 4229cee4648d7172d539f25c68dad9764ecfbb05..7d0f935fd45c2e95302061a4e61d9f4fdaac2896 100644
--- a/platform.cc
+++ b/platform.cc
@@ -386,6 +386,15 @@ bool Platform::SetOwnershipRecursive(const std::string& directory,
   return true;
 }
 
+bool Platform::SetPermissions(const std::string& path, mode_t new_perms) {
+  if (chmod(path.c_str(), new_perms)) {
+    PLOG(ERROR) << "chmod() of " << path.c_str() << " to " << new_perms
+                << " failed.";
+    return false;
+  }
+  return true;
+}
+
 int Platform::SetMask(int new_mask) {
   return umask(new_mask);
 }