ES6 conversion main_window_component.js

Bug: 778674
Change-Id: I548cc05d2ac3ad1c6fc42ebbd86142d031f09883
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1757714
Reviewed-by: François Degros <fdegros@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Jeremie Boulic <jboulic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688410}