Fix ItemSeparatorComponent's leadingItem prop not being updated (#25114)

Summary:
Fix https://github.com/facebook/react-native/issues/24592

Just added a `getDerivedStateFromProps`, similar to what `VirtualizedSectionList` already does: 18fededae0/Libraries/Lists/VirtualizedSectionList.js (L470-L492)

## Changelog

[General] [Fixed] - Fix ItemSeparatorComponent's leadingItem prop not being updated
Pull Request resolved: https://github.com/facebook/react-native/pull/25114

Differential Revision: D15602460

Pulled By: cpojer

fbshipit-source-id: b16a82912fd746a956f6aa360d18ade53357f634

Libraries/Lists/VirtualizedList.js

@@ -1667,8 +1667,7 @@ class VirtualizedList extends React.PureComponent<Props, State> {
   }
 }
 
-class CellRenderer extends React.Component<
-  {
+type CellRendererProps = {
   CellRendererComponent?: ?React.ComponentType<any>,
   ItemSeparatorComponent: ?React.ComponentType<*>,
   cellKey: string,
@@ -1686,8 +1685,18 @@ class CellRenderer extends React.Component<
     ListItemComponent?: ?(React.ComponentType<any> | React.Element<any>),
   },
   prevCellKey: ?string,
-  },
-  $FlowFixMeState,
+};
+
+type CellRendererState = {
+  separatorProps: $ReadOnly<{|
+    highlighted: boolean,
+    leadingItem: ?Item,
+  |}>,
+};
+
+class CellRenderer extends React.Component<
+  CellRendererProps,
+  CellRendererState,
 > {
   state = {
     separatorProps: {
@@ -1702,6 +1711,18 @@ class CellRenderer extends React.Component<
     }),
   };
 
+  static getDerivedStateFromProps(
+    props: CellRendererProps,
+    prevState: CellRendererState,
+  ): ?CellRendererState {
+    return {
+      separatorProps: {
+        ...prevState.separatorProps,
+        leadingItem: props.item,
+      },
+    };
+  }
+
   getChildContext() {
     return {
       virtualizedCell: {