Update Chomium's build files to work w/ latest GN binaries.

build/config/mips.gni

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # MIPS arch variant.
-if (cpu_arch == "mipsel") {
+if (current_cpu == "mipsel") {
   declare_args() {
     mips_arch_variant = "r1"
   }
-} else if (cpu_arch == "mips64el") {
+} else if (current_cpu == "mips64el") {
   if (is_android) {
     declare_args() {
       mips_arch_variant = "r6"
     }
   } else {
     declare_args() {
       mips_arch_variant = "r2"
     }
   }
 }