Fix x64 android_tools target compile failure.

build/secondary/third_party/android_tools/BUILD.gn

Index: build/secondary/third_party/android_tools/BUILD.gn
diff --git a/build/secondary/third_party/android_tools/BUILD.gn b/build/secondary/third_party/android_tools/BUILD.gn
index fa2f67ab93589e27db7daae4c7e07e66dc524ed4..c974f8b5a6deedd314633d0361a9b5093e660a1a 100644
--- a/build/secondary/third_party/android_tools/BUILD.gn
+++ b/build/secondary/third_party/android_tools/BUILD.gn
@@ -6,6 +6,7 @@ import("//build/config/android/rules.gni")
 
 config("cpu_features_include") {
   include_dirs = [ "ndk/sources/android/cpufeatures" ]
+  cflags = [ "-Wno-unused-function" ]

[Nico, 2016/01/05 19:42:36]

That's not correct, for two reasons:

1. This is listed as a public_configs below and is applied to everything
depending on this target.

2. cflags in configs are applied in config order in gn. Since cpu_features below
appends no_chromium_code after this is set in public_configs, I think the -Wall
in no_chromium_code will be ordered after -Wno-unused-function, and in
"-Wno-unused-function -Wall" the later Wall wins over the earlier
Wno-unused-function.

You need to make a new

config("cpu_features_warnings") {
  if (is_clang) {
    cflags = [ "-Wno-unused-function" ]
  }
}

and then add that to configs in cpu_features (with a comment like
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/zlib/B...)

[Kibeom Kim (inactive), 2016/01/05 20:41:22]

Done.

 }
 
 # This is the GN version of