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: 6bda02b51d
>
> 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: 7c8786ae8fTBR=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}
native configuration.
Added a getConfiguration getter method to RTCPeerConnection to return
the RTCConfiguration.
BUG=webrtc:7431
Review-Url: https://codereview.webrtc.org/2790833002
Cr-Commit-Position: refs/heads/master@{#17517}
