Fix ScrollView getInnerViewNode and getInnerViewRef ref methods (#30588)

Summary:
Currently ScrollView ref's `getInnerViewNode` and `getInnerViewRef` are unbound and don't work as expected returning empty values. The origin of this problem probably is issued by d2f314a

Working example of the problem is available here: https://github.com/vshab/RNGetInnerViewBug

This PR binds getInnerViewNode and getInnerViewRef to ScrollView and adds test checking the value of getInnerViewRef.

Before:
![Simulator Screen Shot - iPhone 11 - 2020-12-15 at 02 07 03](https://user-images.githubusercontent.com/6755908/102149544-c7df4900-3e7f-11eb-89de-de39a28fbdb3.png)
After:
![Simulator Screen Shot - iPhone 11 - 2020-12-15 at 01 49 31](https://user-images.githubusercontent.com/6755908/102149559-d168b100-3e7f-11eb-8b27-031c9e43112c.png)

## Changelog

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[JavaScript] [Fixed] - ScrollView: Fix `getInnerViewNode` and `getInnerViewRef` ref methods

Pull Request resolved: #30588

Test Plan:
1. The test checking the value of getInnerViewRef is added.
2. Example app demonstrating the problem is provided with before/after screenshots.

Reviewed By: TheSavior, nadiia

Differential Revision: D25916413

Pulled By: kacieb

fbshipit-source-id: bf18079682be7c647b8715bd0f36cf84953abcfa

Author: vshab

Libraries/Components/ScrollView/ScrollView.js

@@ -808,13 +808,13 @@ class ScrollView extends React.Component<Props, State> {
     return ReactNative.findNodeHandle(this._scrollViewRef);
   };
 
-  getInnerViewNode(): ?number {
+  getInnerViewNode: () => ?number = () => {
     return ReactNative.findNodeHandle(this._innerViewRef);
-  }
+  };
 
-  getInnerViewRef(): ?React.ElementRef<typeof View> {
+  getInnerViewRef: () => ?React.ElementRef<typeof View> = () => {
     return this._innerViewRef;
-  }
+  };
 
   getNativeScrollRef: () => ?React.ElementRef<HostComponent<mixed>> = () => {
     return this._scrollViewRef;

Libraries/Components/ScrollView/__tests__/ScrollView-test.js

@@ -48,4 +48,20 @@ describe('<ScrollView />', () => {
       jest.fn().constructor,
     );
   });
+  it('getInnerViewRef for case where it returns a native view', () => {
+    jest.resetModules();
+    jest.unmock('../ScrollView');
+
+    const scrollViewRef = React.createRef(null);
+
+    ReactTestRenderer.create(<ScrollView ref={scrollViewRef} />);
+
+    const innerViewRef = scrollViewRef.current.getInnerViewRef();
+
+    // This is checking if the ref acts like a host component. If we had an
+    // `isHostComponent(ref)` method, that would be preferred.
+    expect(innerViewRef.measure).toBeInstanceOf(jest.fn().constructor);
+    expect(innerViewRef.measureLayout).toBeInstanceOf(jest.fn().constructor);
+    expect(innerViewRef.measureInWindow).toBeInstanceOf(jest.fn().constructor);
+  });
 });