GN: remove clang flag for NaCl build.

NaCl's version of clang is slightly older than the hermetic version used
in Chrome, and does not support this flag.  NaCl's GN build is not live
in the Chrome tree, yet, but this flag breaks local work to bring it up.

BUG=https://code.google.com/p/chromium/issues/detail?id=401761

Review URL: https://codereview.chromium.org/811163003

Cr-Original-Commit-Position: refs/heads/master@{#308870}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 395afdd8d2cb172a0003816d33eb5ec785ea2aa7

config/compiler/BUILD.gn

@@ -762,10 +762,18 @@ config("default_warnings") {
 
         # TODO(thakis): Remove, http://crbug.com/263960
         "-Wno-reserved-user-defined-literal",
-
-        # TODO(hans): Get this cleaned up.
-        "-Wno-inconsistent-missing-override",
       ]
+
+      # NaCl's Clang compiler and Chrome's hermetic Clang compiler will almost
+      # always have different versions. Certain flags may not be recognized by
+      # one version or the other.
+      if (!is_nacl) {
+        # Flags NaCl does not recognize.
+        cflags += [
+          # TODO(hans): Get this cleaned up.
+          "-Wno-inconsistent-missing-override",
+        ]
+      }
     }
     if (gcc_version >= 48) {
       cflags_cc += [