webview: Encode input messages in base64.

Fixes #2505, #2538, #2558.

When using `postMessage` to communicate with the WebView, depending
on the content of the message we send, an exception might be thrown.

A minimal but reliable way to reproduce the issue is to send the
string `'%22'`.  Other character combinations might also cause issues.

To work around the issue, we encode our data in the nice boring
characters of base64 so that it doesn't get reinterpreted.

---

More details on how React Native sends messages to the WebView:

The logic is different on iOS vs. Android.  This explains why the
issue this is fixing is Android-specific.

On iOS, a custom navigation scheme is used to pass the data into the
webview; search the RN source for `RCTJSNavigationScheme`, and see the
implementation of `webViewDidFinishLoad` in `RTCWebView.m`.

The Android messaging happens in `receiveCommand` in
`ReactWebViewManager.java`, by navigating to a URL of the form
`javascript:(.....)`, which is a standard way of injecting JavaScript
into web pages.

The issue comes from the fact that since Android 4.4, `loadUrl` does
a URL decode on the string passed to it:
  https://issuetracker.google.com/issues/36995865

See #2854 for links to upstream RN bug reports and PRs.

[greg: lightly revised commit message; found better reference for the
 `loadUrl` issue]

src/webview/MessageListWeb.js

@@ -10,6 +10,7 @@ import getHtml from './html/html';
 import renderMessagesAsHtml from './html/renderMessagesAsHtml';
 import { getInputMessages } from './webViewHandleUpdates';
 import * as webViewEventHandlers from './webViewEventHandlers';
+import { base64Utf8Encode } from '../utils/encoding';
 
 export default class MessageListWeb extends Component<Props> {
   context: Context;
@@ -29,7 +30,7 @@ export default class MessageListWeb extends Component<Props> {
 
   sendMessages = (messages: WebviewInputMessage[]): void => {
     if (this.webview && messages.length > 0) {
-      this.webview.postMessage(JSON.stringify(messages), '*');
+      this.webview.postMessage(base64Utf8Encode(JSON.stringify(messages)), '*');
     }
   };
 

src/webview/js/generatedEs3.js

@@ -228,7 +228,8 @@ var messageHandlers = {
 document.addEventListener('message', function (e) {
   scrollEventsDisabled = true;
 
-  var messages = JSON.parse(e.data);
+  var decodedData = decodeURIComponent(escape(window.atob(e.data)));
+  var messages = JSON.parse(decodedData);
   messages.forEach(function (msg) {
     messageHandlers[msg.type](msg);
   });

src/webview/js/js.js

@@ -252,7 +252,8 @@ const messageHandlers = {
 document.addEventListener('message', e => {
   scrollEventsDisabled = true;
   // $FlowFixMe
-  const messages: WebviewInputMessage[] = JSON.parse(e.data);
+  const decodedData = decodeURIComponent(escape(window.atob(e.data)));
+  const messages: WebviewInputMessage[] = JSON.parse(decodedData);
   messages.forEach((msg: WebviewInputMessage) => {
     // $FlowFixMe
     messageHandlers[msg.type](msg);