Ensure WebViews render about:blank correctly

Summary:
@public

This diff fixes two bugs:
1. When you load a `<WebView/>` with an HTML source string, HTML source doesn't render in the `<WebView/>`. Instead, we get this warning: https://pxl.cl/grz3. Here's what the above page should look like when correctly rendered: https://pxl.cl/grzt

2. Furthermore, you render a blank `<WebView/>` (i.e: with no source prop), it should display a blank page. Instead, we get this warning: https://pxl.cl/grz3

**Bugfix:**
One solution I found was to ensure that `about:blank` is always whitelisted. That way, we don't ever abort navigations to blank pages, which occur when we do:

```ObjectiveC
/** Line 134 in RCTWebView.m */
[_webView loadHTMLString:@"" baseURL:nil];
```

and

```ObjectiveC
/** Line 115 in RCTWebView.m */
if (html) {
  NSURL *baseURL = [RCTConvert NSURL:source[@"baseUrl"]];
  if (!baseURL) {
    baseURL = [NSURL URLWithString:@"about:blank"];
  }
  [_webView loadHTMLString:html baseURL:baseURL];
  return;
}
```

Reviewed By: yungsters, mmmulani

Differential Revision: D9259852

fbshipit-source-id: e1b9673fcd8c3d0df77308df8c4a632a2b596bfb

Libraries/Components/WebView/WebView.ios.js

@@ -435,9 +435,10 @@ class WebView extends React.Component {
 
     const viewManager = nativeConfig.viewManager || RCTWebViewManager;
 
-    const compiledWhitelist = (this.props.originWhitelist || []).map(
-      WebViewShared.originWhitelistToRegex,
-    );
+    const compiledWhitelist = [
+      'about:blank',
+      ...(this.props.originWhitelist || []),
+    ].map(WebViewShared.originWhitelistToRegex);
     const onShouldStartLoadWithRequest = (event: Event) => {
       let shouldStart = true;
       const {url} = event.nativeEvent;