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JS Modules for the following files in ui/file_manger/video_player/js:
- background.js
- error_util.js
- test_util.js

Add the main_background.m.js to act as the entry point for the
background page.

Bug: 1133186
Change-Id: I0c1a205047c85f70e4629d5a3f52ddfcfcbedd7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2596467
Commit-Queue: Jeremie Boulic <jboulic@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837950}