Fix libvpx iOS ARM GN build.

third_party/libvpx/BUILD.gn

Index: third_party/libvpx/BUILD.gn
diff --git a/third_party/libvpx/BUILD.gn b/third_party/libvpx/BUILD.gn
index ea4323b2d650fab5e7494a9c6f003f5057b058b3..3ecb8319630e4b0fbe64a3b697a116e48232ed34 100644
--- a/third_party/libvpx/BUILD.gn
+++ b/third_party/libvpx/BUILD.gn
@@ -239,10 +239,16 @@ if (current_cpu == "arm") {
     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",