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

tools-webrtc/ios/client.webrtc/iOS32_Debug.json

 {
   "comments": [
-    "Builder for 32-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 32-bit devices."
   ],
   "xcode version": "8.0",
   "gn_args": [
+    "goma_dir=\"$(goma_dir)\"",
+    "ios_enable_code_signing=false",
+    "is_component_build=false",
     "is_debug=true",
     "target_cpu=\"arm\"",
+    "target_os=\"ios\"",
     "use_goma=true"

[ehmaldonado_webrtc, 2017/02/15 20:15:36]

I think you might be missing "symbol_level=1"
From 
'iOS32 Debug'
 ->
'ios_debug_static_bot_arm_device'
 ->
'debug_static_bot'
 ->
'minimal_symbols'
 ->
'symbol_level=1'

I don't know if it matters.

[kjellander_webrtc, 2017/02/15 20:28:27]

Good catch. I don't think it matters. Chrome bots don't set it
(https://cs.chromium.org/chromium/src/ios/build/bots/chromium.mac/ios-device.json)
and the default is 0
(https://cs.chromium.org/chromium/src/build/config/compiler/compiler.gni?rcl=7...)
which means no symbols. 

It will actually be interesting to change to an identical config as Chromium,
since we have some weird issues that only happens to us, not them.
I've updated the CL description to make it clear that we're changing this.

   ],
   "additional_compile_targets": [ "all" ],
   "configuration": "Debug",
   "sdk": "iphoneos10.0",
   "tests": [
   ]
 }