Fix crdmg so that it actually works.

After changing to the __builtin_available() syntax in
51938fff, crdmg's sandbox blocked
reading the SystemVersion.plist, which caused crdmg to not properly
detect the OS version.

Bug: 752128
Change-Id: I2d1035c4de8c2f0559ccabc06eaf046da2f6522b
Reviewed-on: https://chromium-review.googlesource.com/600129Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491759}

chrome/utility/safe_browsing/mac/crdmg.cc

@@ -89,10 +89,10 @@ int SafeDMG::Main(int argc, const char* argv[]) {
   if (argc == 3 && !PrepareUnpack(argv[2]))
     return EXIT_FAILURE;
 
-  if (!EnableSandbox())
-    return EXIT_FAILURE;
-
   if (__builtin_available(macOS 10.10, *)) {
+    if (!EnableSandbox())
+      return EXIT_FAILURE;
+
     if (!ParseDMG())
       return EXIT_FAILURE;
   } else {