[iOS/GN] Allow overriding parameters of "ios_app_bundle" in "test" template.

testing/test.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.
 
 # ==============================================================================
 # TEST SETUP
 # ==============================================================================
 
 template("_gen_isolate") {
   testonly = true
@@ skipping 274 matching lines @@
       ]
       sources = [
         "//testing/gtest_ios/Default.png",
       ]
       outputs = [
         "{{bundle_resources_dir}}/{{source_file_part}}",
       ]
     }
 
     ios_app_bundle(_test_target) {
-      # TODO: Make this configurable and only provide a default that can be
-      # overridden.
-      info_plist = "//testing/gtest_ios/unittest-Info.plist"
-      entitlements_path = "//testing/gtest_ios"
-      code_signing_identity = ""
       testonly = true
 
+      # See above call.
+      set_sources_assignment_filter([])
+      forward_variables_from(invoker, "*", [ "testonly" ])
+
+      # Provide sensible defaults in case invoker did not define any of those
+      # required variables.
+      if (!defined(info_plist)) {
+        info_plist = "//testing/gtest_ios/unittest-Info.plist"
+      }
+      if (!defined(entitlements_path)) {
+        entitlements_path = "//testing/gtest_ios"
+      }
+      if (!defined(code_signing_identity)) {
+        code_signing_identity = ""
+      }
+
       # TODO(crbug.com/603102): remove this once gyp support is dropped and all
       # application uses the target name as value for BUNDLE_ID_TEST_NAME.
       if (defined(invoker.app_name)) {
         app_name = invoker.app_name
       } else {
         app_name = target_name
       }
 
-      extra_substitutions = [ "BUNDLE_ID_TEST_NAME=$app_name" ]
-
-      # See above call.
-      set_sources_assignment_filter([])
-
-      forward_variables_from(invoker, "*")
+      if (!defined(extra_substitutions)) {
+        extra_substitutions = []
+      }
+      extra_substitutions += [ "BUNDLE_ID_TEST_NAME=$app_name" ]
 
       if (!defined(deps)) {
         deps = []
       }
       deps += [
         ":$_resources_bundle_data",
 
         # All shared libraries must have the sanitizer deps to properly link in
         # asan mode (this target will be empty in other cases).
         "//build/config/sanitizers:deps",
@@ skipping 27 matching lines @@
     if (defined(invoker.output_name) && target_name != invoker.output_name) {
       group("${invoker.output_name}_run") {
         testonly = true
         deps = [
           ":${invoker.target_name}",
         ]
       }
     }
   }
 }