Fix share preference plugin layout flicker (#3903)

Summary:
The share preference plugin page flashes when INSPECTOR's width is too large.

## Changelog

Add 'overflowX: 'hidden'' to FlexRow.

Pull Request resolved: https://github.com/facebook/flipper/pull/3903

Test Plan: Pass the test on MAC.

Reviewed By: passy

Differential Revision: D38784840

Pulled By: mweststrate

fbshipit-source-id: 4ca3fc17ccc66767e75ce0f9ab7ad71741dbf15e

desktop/plugins/public/shared_preferences/src/index.tsx

@@ -180,7 +180,7 @@ export function Component() {
           onChange={instance.setSelectedPreferences}
         />
       </Heading>
-      <FlexRow grow scrollable>
+      <FlexRow grow scrollable style={{overflowX: 'hidden'}}>
         <InspectorColumn>
           <Heading>Inspector</Heading>
           <ManagedDataInspector