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

Remove all uses of retain/release and NSAutoreleasePool.

This makes transformation to Bazel easier.

This CL subsumes https://codereview.webrtc.org/2778163002 and depends on https://codereview.webrtc.org/2784483002/

BUG=webrtc:6412

Review-Url: https://codereview.webrtc.org/2781713004
Cr-Commit-Position: refs/heads/master@{#17780}
diff --git a/webrtc/webrtc.gni b/webrtc/webrtc.gni
index 6f902c9..e47ac08 100644
--- a/webrtc/webrtc.gni
+++ b/webrtc/webrtc.gni
@@ -211,6 +211,10 @@
 # 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