[Files app] ES6 class for ui/file_manager/file_manager/foreground/js/search_controller.js

Bug: 778674
Change-Id: Id6a3c5a48af4085a74282e7f3f89cc95dd426e0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1630075
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: François Degros <fdegros@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663476}