WKWebView inits itself twice

I added `NSLog(@"I am %x", self);` to the `- (instancetype)initWithProcessPool:(WKProcessPool *)processPool` block. In my project (note: macOS fork of this codebase; up-to-date with 1.20.0), the line logs twice in fast succession, with two different id values:

```
2018-05-31 23:30:52.277716+0100 MyApp[39471:630092] I am 1738780
2018-05-31 23:31:33.413092+0100 MyApp[39471:630092] I am 152f3b0
```

Note: I have only constructed one `` component in the React Native JS code, and adding the same message to the `dealloc()` block shows that it is not being dealloced soon after either init.

Also, I am using `react-navigation`, which may complicate things.

Does anyone else experience this (on the iOS master)?