[iOS/GN] Fix the compilation of "all" on iOS with gn.

mojo/edk/test/BUILD.gn

Index: mojo/edk/test/BUILD.gn
diff --git a/mojo/edk/test/BUILD.gn b/mojo/edk/test/BUILD.gn
index 791d62c00b6a37c69ec45b28d8b6ec1d830f6b94..cd56fb8e398967adaa8a098aa8129420b7baea95 100644
--- a/mojo/edk/test/BUILD.gn
+++ b/mojo/edk/test/BUILD.gn
@@ -9,8 +9,6 @@ source_set("test_support") {
   sources = [
     "mojo_test_base.cc",
     "mojo_test_base.h",
-    "multiprocess_test_helper.cc",
-    "multiprocess_test_helper.h",
     "scoped_ipc_support.cc",
     "scoped_ipc_support.h",
     "test_utils.h",
@@ -18,6 +16,13 @@ source_set("test_support") {
     "test_utils_win.cc",
   ]
 
+  if (!is_ios) {
+    sources += [
+      "multiprocess_test_helper.cc",
+      "multiprocess_test_helper.h",
+    ]
+  }
+
   deps = [
     "//base",
     "//base/test:test_support",