Reland of GN: Enable ARC for Mac and iOS in rtc_* templates (patchset #1 id:1 of https://codereview.webrtc.org/2827223003/ )

Reason for revert:
Relanding after fixing ARC issue.

Original issue's description:
> Revert of GN: Enable ARC for Mac and iOS in rtc_* templates (patchset #3 id:40001 of https://codereview.webrtc.org/2781713004/ )
>
> Reason for revert:
> Breaks mac build
>
> Original issue's description:
> > 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}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/6bda02b51dc00401d2a0d04be996ed10e5acac47
>
> TBR=kjellander@webrtc.org,magjed@webrtc.org,stefan@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:6412
>
> Review-Url: https://codereview.webrtc.org/2827223003
> Cr-Commit-Position: refs/heads/master@{#17784}
> Committed: https://chromium.googlesource.com/external/webrtc/+/7c8786ae8f2d8481f74b0ab16eb91a8101fe51dc

TBR=kjellander@webrtc.org,magjed@webrtc.org,stefan@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:6412

Review-Url: https://codereview.webrtc.org/2834273002
Cr-Commit-Position: refs/heads/master@{#17836}
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