iOS: Update MB and JSON configs + enable Goma

webrtc/build/ios/client.webrtc/iOS64_Release.json

 {
   "owners": [
     "kjellander"
   ],
   "comments": [
-    "Builder for 64-bit devices."
+    "Builder for 64-bit devices.",
+    "GYP_DEFINES and gn_args in this file are only used during runhooks; ",
+    "webrtc/build/mb_config.pyl decides the generated projects that are compiled."
   ],
   "xcode version": "7.0",
-  "GYP_DEFINES": {
-    "chromium_ios_signing": "0",
-    "target_arch": "arm64"
-  },
+  "GYP_DEFINES": [
+    "use_goma=1",
+    "gomadir=$(goma_dir)",
+    "chromium_ios_signing=0",
+    "component=static_library",
+    "target_arch=arm64"
+  ],
+  "gn_args": [
+    "is_debug=false",
+    "target_os=\"ios\"",
+    "ios_enable_code_signing=false",
+    "is_component_build=false",
+    "use_goma=true",
+    "goma_dir=\"$(goma_dir)\"",
+    "target_cpu=\"arm64\""
+  ],
+  "mb_type": "gyp",
   "compiler": "ninja",
   "configuration": "Release",
   "sdk": "iphoneos9.0",
   "tests": [
   ]
 }