Avoid UI jankiness caused by triggering selection scan too frequently.

BUG=671517
TEST=manual test by the repro step noted in the bug
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2589823002
Cr-Commit-Position: refs/heads/master@{#439702}

ui/file_manager/file_manager/foreground/js/import_controller.js

@@ -1211,7 +1211,7 @@ importer.RuntimeControllerEnvironment.prototype.addDirectoryChangedListener =
 importer.RuntimeControllerEnvironment.prototype.addSelectionChangedListener =
     function(listener) {
   this.selectionHandler_.addEventListener(
-      FileSelectionHandler.EventType.CHANGE,
+      FileSelectionHandler.EventType.CHANGE_THROTTLED,
       listener);
 };