Reland of iOS: Use JSON for GN configuration instead of MB + remove symbols

tools-webrtc/ios/client.webrtc/iOS64_Release.json

Index: tools-webrtc/ios/client.webrtc/iOS64_Release.json
diff --git a/tools-webrtc/ios/client.webrtc/iOS64_Release.json b/tools-webrtc/ios/client.webrtc/iOS64_Release.json
index 1cc6e2ae0295bfb679fd14cd209c71cc454409c8..71242ae740f435141e17fa084b8c81cb91cf3ab2 100644
--- a/tools-webrtc/ios/client.webrtc/iOS64_Release.json
+++ b/tools-webrtc/ios/client.webrtc/iOS64_Release.json
@@ -1,14 +1,16 @@
 {
   "comments": [
-    "Builder for 64-bit devices.",
-    "gn_args in this file is only read by the buildbot recipe; ",
-    "webrtc/build/mb_config.pyl contains the actual build configuration.",
-    "TODO(kjellander): remove gn_args when the recipe is updated."
+    "Builder for 64-bit devices."
   ],
   "xcode version": "8.0",
   "gn_args": [
+    "goma_dir=\"$(goma_dir)\"",
+    "dcheck_always_on=true",
+    "ios_enable_code_signing=false",
+    "is_component_build=false",
     "is_debug=false",
     "target_cpu=\"arm64\"",
+    "target_os=\"ios\"",
     "use_goma=true"
   ],
   "additional_compile_targets": [ "all" ],