Bug: Input leaks fiber nodes in recent versions of chromium

<!-- Please provide a clear and concise description of what the bug is. Include screenshots if needed. Please test using the latest version of the relevant React packages to make sure your issue has not already been fixed. --> React version: ^16.13.1 ## Steps To Reproduce 1. Mount an input component 2. Type anything 3. Unmount input component Small code sample: ``` const InputWrapper = () => { const Siblings = React.useMemo(() => new Array(1000).fill(1).map((_,i) => <div key={i}/>), []); return ( <div> <input /> {Siblings} </div> ); }; export const Demo = () => { const [show, setShow] = React.useState(false); const handleShow = React.useCallback(() => { setShow((prev) => !prev); }, []); const closeRef = React.useRef<number>(0); const handleShowMany = React.useCallback(() => { closeRef.current = 0; const id = setInterval(() => { setShow(prev => !prev); if (closeRef.current++ > 10000) { clearInterval(id); } }, 1); }, [setShow]) return ( <div> <button onClick={handleShow}>{show ? "hide" : "show"}</button> <button onClick={handleShowMany}>{closeRef.current}</button> {show && <InputWrapper />} </div> ); }; ``` ## The current behavior Currently when mounting the input controller for the first time, a small increase in fiber nodes occurs, and after typing in the input box, seemingly the entire React fiber tree is leaked. When mounting/unmounting the input again, there is no clean up (even after thousands of iterations). ## The expected behavior The fiber nodes to be cleaned up after unmounting the input element. Comparison memory+fiber nodes on Chrome 69 vs Chrome 86: **Chrome 69**:  **Chrome 86**:  **Note the 600kb increase in memory per …