ninja/mac: Try to fix 'IOError: [Errno 2] No such file or directory' error.

Happening on this Blink bot:

This probably broke when we changed the generator to write ninja files after
their contents are known -- because of that the folder doesn't exist yet
when the per-arch files are written.

Review URL:

git-svn-id: 78cadc50-ecff-11dd-a971-7dbc132099af
diff --git a/pylib/gyp/generator/ b/pylib/gyp/generator/
index 1d4b913..368993c 100644
--- a/pylib/gyp/generator/
+++ b/pylib/gyp/generator/
@@ -396,9 +396,9 @@
       if len(self.archs) > 1:
         self.arch_subninjas = dict(
             (arch, ninja_syntax.Writer(
-                open(os.path.join(self.toplevel_build,
-                                  self._SubninjaNameForArch(arch)),
-                     'w')))
+                OpenOutput(os.path.join(self.toplevel_build,
+                                        self._SubninjaNameForArch(arch)),
+                           'w')))
             for arch in self.archs)
     # Compute predepends for all rules.