Fix nan definition on GN Android build.

This resulted in a double -D definition.

TBR=scottmg

Review URL: https://codereview.chromium.org/132903005

git-svn-id: http://src.chromium.org/svn/trunk/src/build@244203 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

config/compiler/BUILD.gn

@@ -291,7 +291,7 @@ config("runtime_library") {
       # Work around incompatibilities between bionic and clang headers.
       defines += [
         "__compiler_offsetof=__builtin_offsetof",
-        "-Dnan=__builtin_nan",
+        "nan=__builtin_nan",
       ]
     }