GN: Fix some build errors for iOS.

webrtc/test/BUILD.gn

Index: webrtc/test/BUILD.gn
diff --git a/webrtc/test/BUILD.gn b/webrtc/test/BUILD.gn
index 54694b8b646d5899d64656fee23370769d07bb4b..af9516f43426c54a3eada81e3fcfc7c7e57dd8f5 100644
--- a/webrtc/test/BUILD.gn
+++ b/webrtc/test/BUILD.gn
@@ -59,6 +59,7 @@ source_set("test_support") {
     "testsupport/frame_reader.h",
     "testsupport/frame_writer.cc",
     "testsupport/frame_writer.h",
+    "testsupport/iosfileutils.mm",
     "testsupport/mock/mock_frame_reader.h",
     "testsupport/mock/mock_frame_writer.h",
     "testsupport/packet_reader.cc",
@@ -76,6 +77,10 @@ source_set("test_support") {
     "//testing/gtest",
   ]
 
+  if (is_ios) {
+    cflags = [ "-fobjc-arc" ]  # CLANG_ENABLE_OBJC_ARC = YES.
+  }
+
   if (is_android) {
     deps += [ "//base:base" ]
   }