Prevents JS Debugger issues with CORS

Summary:
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See #17618

On certain networks, `xip.io` is used, but as the debugger will always using localhost a change upstream has resulted in a CORS issue (see screenshots in #17618). This change ensures that the debugger will always open with whatever configuration.

> This should be merged in as a patch of `0.52.x` as it affects current release, and `0.51.x` didn't have this issue.

Tested locally, could do with the people having the same issue in #17618 testing it out prior to merging.

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/react-native-website, and link to your PR here.)

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  CATEGORY
[----------]        TYPE
[ CLI      ]   [-------------]      LOCATION
[ DOCS     ]   [ BREAKING    ]   [-------------]
[ GENERAL  ]   [ BUGFIX      ]   [-{Component}-]
[ INTERNAL ]   [ ENHANCEMENT ]   [ {File}      ]
[ IOS      ]   [ FEATURE     ]   [ {Directory} ]   |-----------|
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[CATEGORY] [TYPE] [LOCATION] - MESSAGE

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 [IOS] [BREAKING] [FlatList] - Change a thing that breaks other things
 [ANDROID] [BUGFIX] [TextInput] - Did a thing to TextInput
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 [INTERNAL] [FEATURE] [./scripts] - Added thing to script that nobody will see
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[CLI] [BUGFIX] [local-cli/server/middleware/getDevToolsMiddleware.js] - Prevents JS Debugger issues with CORS
Closes https://github.com/facebook/react-native/pull/17720

Differential Revision: D6828205

Pulled By: hramos

fbshipit-source-id: 7e5d43db9faf7edc9444ba4214aca1a18e25dbd2

local-cli/server/middleware/getDevToolsMiddleware.js

@@ -14,8 +14,8 @@ const launchChrome = require('../util/launchChrome');
 
 const {exec} = require('child_process');
 
-function launchChromeDevTools(port, args = '') {
-  var debuggerURL = 'http://localhost:' + port + '/debugger-ui' + args;
+function launchChromeDevTools(host, args = '') {
+  var debuggerURL = 'http://' + host + '/debugger-ui' + args;
   console.log('Launching Dev Tools...');
   launchChrome(debuggerURL);
 }
@@ -25,7 +25,7 @@ function escapePath(pathname) {
   return '"' + pathname + '"';
 }
 
-function launchDevTools({port, projectRoots}, isChromeConnected) {
+function launchDevTools({host, projectRoots}, isChromeConnected) {
   // Explicit config always wins
   var customDebugger = process.env.REACT_DEBUGGER;
   if (customDebugger) {
@@ -39,12 +39,13 @@ function launchDevTools({port, projectRoots}, isChromeConnected) {
     });
   } else if (!isChromeConnected()) {
     // Dev tools are not yet open; we need to open a session
-    launchChromeDevTools(port);
+    launchChromeDevTools(host);
   }
 }
 
 module.exports = function(options, isChromeConnected) {
   return function(req, res, next) {
+    var host = req.headers.host;
     if (req.url === '/launch-safari-devtools') {
       // TODO: remove `console.log` and dev tools binary
       console.log(
@@ -62,7 +63,7 @@ module.exports = function(options, isChromeConnected) {
       launchDevTools(options, isChromeConnected);
       res.end('OK');
     } else if (req.url === '/launch-js-devtools') {
-      launchDevTools(options, isChromeConnected);
+      launchDevTools({...options, host}, isChromeConnected);
       res.end('OK');
     } else {
       next();