Fix libvpx iOS ARM GN build.

third_party/libvpx/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.
 
 import("//build/config/arm.gni")
 import("//build/config/android/config.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//third_party/libvpx/libvpx_srcs.gni")
 import("//third_party/yasm/yasm_assemble.gni")
 
@@ skipping 221 matching lines @@
     if (cpu_arch_full == "arm-neon") {
       sources = libvpx_srcs_arm_neon_assembly
     } else if (cpu_arch_full == "arm-neon-cpu-detect") {
       sources = libvpx_srcs_arm_neon_cpu_detect_assembly
     } else {
       sources = libvpx_srcs_arm_assembly
     }
     outputs = [
       "$target_gen_dir/{{source_name_part}}.S",
     ]
+    if (is_ios) {
+      ads2gas_script =
+          "//third_party/libvpx/source/libvpx/build/make/ads2gas_apple.pl"
+    } else {
+      ads2gas_script =
+          "//third_party/libvpx/source/libvpx/build/make/ads2gas.pl"
+    }
     args = [
       "-s",
-      rebase_path("//third_party/libvpx/source/libvpx/build/make/ads2gas.pl",
-                  root_build_dir),
+      rebase_path(ads2gas_script, root_build_dir),
       "-i",
       "{{source}}",
       "-o",
       rebase_path("$target_gen_dir/{{source_name_part}}.S"),
     ]
   }
 
   static_library("libvpx_assembly_arm") {
     sources = get_target_outputs(":convert_arm_assembly")
     configs -= [ "//build/config/compiler:compiler_arm_fpu" ]
@@ skipping 59 matching lines @@
   }
   if (is_android) {
     deps += [ "//third_party/android_tools:cpu_features" ]
   }
   if (current_cpu == "arm") {
     deps += [ ":libvpx_assembly_arm" ]
   }
 
   public_configs = [ ":libvpx_external_config" ]
 }