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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]

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