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/sdk/BUILD.gn b/webrtc/sdk/BUILD.gn
index 38c5b68..c27811a 100644
--- a/webrtc/sdk/BUILD.gn
+++ b/webrtc/sdk/BUILD.gn
@@ -31,10 +31,7 @@
     deps = [
       "../base:rtc_base",
     ]
-    configs += [
-      "..:common_objc",
-      "//build/config/compiler:enable_arc",
-    ]
+    configs += [ "..:common_objc" ]
     public_configs = [ ":rtc_sdk_common_objc_config" ]
     sources = [
       "objc/Framework/Classes/NSString+StdString.h",
@@ -230,10 +227,7 @@
       ]
     }
 
-    configs += [
-      "..:common_objc",
-      "//build/config/compiler:enable_arc",
-    ]
+    configs += [ "..:common_objc" ]
 
     public_configs = [ ":rtc_sdk_common_objc_config" ]
 
@@ -373,10 +367,7 @@
         "VideoToolbox.framework",
       ]
 
-      configs += [
-        "..:common_objc",
-        "//build/config/compiler:enable_arc",
-      ]
+      configs += [ "..:common_objc" ]
 
       public_configs = [ ":rtc_sdk_common_objc_config" ]
 
@@ -406,10 +397,7 @@
       "objc/Framework/Classes/h264_video_toolbox_nalu.h",
     ]
 
-    configs += [
-      "//webrtc:common_objc",
-      "//build/config/compiler:enable_arc",
-    ]
+    configs += [ "//webrtc:common_objc" ]
 
     deps = [
       ":rtc_sdk_common_objc",