Files.app: Invalidate ScanResult when the scanned directory is changed.
* Add whenInvalidated method to ScanResult.
* Watch the invalidation in ImportController.
BUG=
420680
TEST=FileManagerJsTest.MediaScannerTest
Committed:
https://crrev.com/9ec42b887a697f656b4df721dbde5cd08104af96
Cr-Commit-Position: refs/heads/master@{#312977}