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

tools-webrtc/ios/tryserver.webrtc/ios64_sim_ios9_dbg.json

 {
   "comments": [
-    "Tests for 64-bit iOS simulators.",
-    "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."
+    "Tests for 64-bit iOS simulators."
   ],
   "xcode version": "8.0",
   "gn_args": [
+    "goma_dir=\"$(goma_dir)\"",
+    "is_component_build=false",
     "is_debug=true",
     "target_cpu=\"x64\"",
+    "target_os=\"ios\"",
     "use_goma=true"
   ],
   "use_analyze": false,
   "additional_compile_targets": [ "all" ],
   "configuration": "Debug",
   "sdk": "iphonesimulator9.0",
   "tests": [
     {
       "include": "common_tests.json",
       "device type": "iPhone 6s",
       "os": "9.0"
     }
   ]
 }