Add Content-Type header to dev server requests (#49042)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49042

Minor fix to align with adjacent `/open-stack-frame` call: https://fburl.com/code/o9rwbmxr.

Changelog: [Internal]

Reviewed By: robhogan

Differential Revision: D68830215

fbshipit-source-id: 87ef0c14bdedd34153014721b85bd24af24c1db7

packages/react-native/Libraries/Core/Devtools/openURLInBrowser.js

@@ -16,6 +16,9 @@ export default function openURLInBrowser(url: string) {
   // $FlowFixMe[unused-promise]
   fetch(getDevServer().url + 'open-url', {
     method: 'POST',
+    headers: {
+      'Content-Type': 'application/json',
+    },
     body: JSON.stringify({url}),
   });
 }

packages/react-native/Libraries/Core/Devtools/symbolicateStackTrace.js

@@ -42,6 +42,9 @@ export default async function symbolicateStackTrace(
   const fetch = global.fetch ?? require('../../Network/fetch');
   const response = await fetch(devServer.url + 'symbolicate', {
     method: 'POST',
+    headers: {
+      'Content-Type': 'application/json',
+    },
     body: JSON.stringify({stack, extraData}),
   });
   return await response.json();