Try again to suppress mac warnings

xfa.gyp

Index: xfa.gyp
diff --git a/xfa.gyp b/xfa.gyp
index a974fcf71615899a1d4abcb04a25b838a5516d4b..e51d4d99464da383334f07b670702e7b2358ece3 100644
--- a/xfa.gyp
+++ b/xfa.gyp
@@ -11,6 +11,14 @@
       # TODO(thestig): Fix all instances, remove this, pdfium:29
       4245, 4310, 4389, 4701, 4702, 4706, 4800,
     ],
+    'variables': {
+      'clang_warning_flags': [
+        # TODO(thestig): Fix all instances, remove this, pdfium:29
+        '-Wno-sign-compare',
+      ],
+      # Make sure Chromium's build/common.gypi doesn't re-add the flag on linux.
+      'cflags_cc!': [ '-Wsign-compare' ],
+    },
   },
   "targets":[
     {