GN: Enable ARC for Mac and iOS in rtc_* templates

webrtc/webrtc.gni

Index: webrtc/webrtc.gni
diff --git a/webrtc/webrtc.gni b/webrtc/webrtc.gni
index 6f902c97bf393c9b27673fc425d2b0bfa5663c63..e47ac08afcca21a6a37267a157a1a60a4f446b00 100644
--- a/webrtc/webrtc.gni
+++ b/webrtc/webrtc.gni
@@ -211,6 +211,10 @@ webrtc_root = get_path_info(".", "abspath")
 # native code is built as part of Chromium.
 rtc_common_configs = [ webrtc_root + ":common_config" ]
 
+if (is_mac || is_ios) {
+  rtc_common_configs += [ "//build/config/compiler:enable_arc" ]
+}
+
 # Global public configuration that should be applied to all WebRTC targets. You
 # normally shouldn't need to include this in your target as it's automatically
 # included when using the rtc_* templates. It set the defines, include paths and