Remove V8_DEPRECATION_WARNINGS for mac/xfa

TBR=ochang@chromium.org

Review URL: https://codereview.chromium.org/1768693002 .
diff --git a/xfa.gyp b/xfa.gyp
index 65d4751..a974fcf 100644
--- a/xfa.gyp
+++ b/xfa.gyp
@@ -701,6 +701,7 @@
           "configurations": {},
           "sources": [],
           "cflags": [ "-Wno-deprecated-declarations" ],
+          "defines!": [ "V8_DEPRECATION_WARNINGS" ],
         }],
       ]
     }