Land prep work to enable NaCl in the Linux x64 GN builds.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index e47ef757b7abc16dcb050d168a15e189ecd87ea3..856d5048565d442c6721cb5aeadef908d7261ed3 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -8,7 +8,9 @@
 # you add a new build file, there must be some path of dependencies from this
 # file to your new one or GN won't know about it.
 
+import("//build/config/features.gni")
 import("//build/config/ui.gni")
+
 if (is_android) {
   import("//build/config/android/config.gni")
 }
@@ -161,6 +163,13 @@ group("root") {
     deps += [ "//ui/wm" ]
   }
 
+  if (enable_nacl && enable_nacl_untrusted) {
+    if (is_linux && cpu_arch == "x64") {
+      # TODO(GYP): Add the right deps for the other architectures.
+      deps += [ "//ppapi:ppapi_cpp_lib(//native_client/build/toolchain/nacl:clang_newlib_x64)" ]
+    }
+  }
+
   if (is_win) {
     deps += [ "//ui/metro_viewer" ]
   }