Make react-devtools server port can be changed

Summary:
This PR make [server port of react-devtools](https://github.com/facebook/react-devtools/blob/master/shells/electron/index.html#L71) can be changed, currently we can set the port yourself and open it, so I think it would be better if it could also be set here.

Another reason is I can debug two RN app (`react-native start --port 8082`), but I cannot let them use react-devtools together, it would be better if I set
different `__REACT_DEVTOOLS_PORT__` in `index.ios.js` and `index.android.js`.
Closes facebook#10522

Differential Revision: D4144011

Pulled By: hramos

fbshipit-source-id: de67931f377092871a0fe92e7d9a18799625217e

Libraries/Core/Devtools/setupDevtools.js

@@ -21,7 +21,8 @@ function setupDevtools() {
   if (Platform.OS === 'android' && NativeModules.AndroidConstants) {
     hostname = NativeModules.AndroidConstants.ServerHost.split(':')[0];
   }
-  var ws = new window.WebSocket('ws://' + hostname + ':8097/devtools');
+  var port = window.__REACT_DEVTOOLS_PORT__ || 8097;
+  var ws = new window.WebSocket('ws://' + hostname + ':' +  port + '/devtools');
   // this is accessed by the eval'd backend code
   var FOR_BACKEND = { // eslint-disable-line no-unused-vars
     resolveRNStyle: require('flattenStyle'),