react-native-webview libdef: Use React$Element with required type argument

This fixes one of the things Flow glosses over and would catch when
we migrate this to a .js.flow file, with an error like this:

  Cannot use `React.Element` [1] without 1 type argument.
    [missing-type-arg]

Without having a particular type in mind to pass, just use the
highest, most general type allowed. See definition [1]:

  /**
   * Type of a React element. React elements are commonly created using JSX
   * literals, which desugar to React.createElement calls (see below).
   */
  declare type React$Element<+ElementType: React$ElementType> = {|
    +type: ElementType,
    +props: React$ElementProps<ElementType>,
    +key: React$Key | null,
    +ref: any,
  |};

[1] https://github.com/facebook/flow/blob/v0.141.0/lib/react.js#L174-L183

Author: chrisbobbe

flow-typed/react-native-webview_v11.x.x.js

@@ -73,7 +73,7 @@ declare module 'react-native-webview/@@react-native' {
      * @platform ios
      */
     takeSnapshot: (
-      view?: 'window' | React$Element<> | number,
+      view?: 'window' | React$Element<React$ElementType> | number,
       options?: {|
         width?: number,
         height?: number,
@@ -1175,12 +1175,12 @@ declare module 'react-native-webview' {
       errorDomain: string | void,
       errorCode: number,
       errorDesc: string,
-    ) => React$Element,
+    ) => React$Element<React$ElementType>,
 
     /**
      * Function that returns a loading indicator.
      */
-    renderLoading?: () => React$Element,
+    renderLoading?: () => React$Element<React$ElementType>,
 
     /**
      * Function that is invoked when the `WebView` scrolls.