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

build/secondary/third_party/libsrtp/BUILD.gn

 # Copyright 2014 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.
 
 declare_args() {
   use_system_libsrtp = false
 }
 
 config("libsrtp_config") {
   defines = [
@@ skipping 27 matching lines @@
     defines += [
       "INLINE=__inline",
       "HAVE_BYTESWAP_METHODS_H",
 
       # All Windows architectures are this way.
       "SIZEOF_UNSIGNED_LONG=4",
       "SIZEOF_UNSIGNED_LONG_LONG=8",
     ]
   }
 
-  if (cpu_arch == "x64" || cpu_arch == "x86" || cpu_arch == "arm") {
+  if (current_cpu == "x64" || current_cpu == "x86" || current_cpu == "arm") {
     defines += [
       # TODO(leozwang): CPU_RISC doesn"t work properly on android/arm
       # platform for unknown reasons, need to investigate the root cause
       # of it. CPU_RISC is used for optimization only, and CPU_CISC should
       # just work just fine, it has been tested on android/arm with srtp
       # test applications and libjingle.
       "CPU_CISC",
     ]
   }
 
-  if (cpu_arch == "mipsel") {
+  if (current_cpu == "mipsel") {
     defines += [ "CPU_RISC" ]
   }
 }
 
 config("system_libsrtp_config") {
   defines = [ "USE_SYSTEM_LIBSRTP" ]
   include_dirs = [ "/usr/include/srtp" ]
 }
 
 if (use_system_libsrtp) {
@@ skipping 260 matching lines @@
         ":srtp_test_stat_driver",
         ":srtp_test_sha1_driver",
         ":srtp_test_kernel_driver",
         ":srtp_test_aes_calc",
         ":srtp_test_rand_gen",
         ":srtp_test_env",
       ]
     }
   }
 }