Added WebView deprecation warning (#22980)

Summary:
Changelog:
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[General] [Deprecated] - Deprecation warning for WebView as it has been extracted from core.
Pull Request resolved: #22980

Differential Revision: D13664505

Pulled By: hramos

fbshipit-source-id: 0cfc06015e77c52bed9eeebd97c8cbca6eacd8ce

Libraries/react-native/react-native-implementation.js

@@ -14,6 +14,7 @@ const invariant = require('invariant');
 
 let showedListViewDeprecation = false;
 let showedSwipeableListViewDeprecation = false;
+let showedWebWiewDeprecation = false;
 
 // Export React, plus some native additions.
 module.exports = {
@@ -167,6 +168,15 @@ module.exports = {
     return require('VirtualizedList');
   },
   get WebView() {
+    if (!showedWebWiewDeprecation) {
+      console.warn(
+        'WebView has been extracted from react-native core and will be removed in a future release. ' +
+          "It can now be installed and imported from 'react-native-webview' instead of 'react-native'. " +
+          'See https://github.com/react-native-community/react-native-webview for more informations.',
+      );
+
+      showedWebWiewDeprecation = true;
+    }
     return require('WebView');
   },