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Background.m.js replaces background.js and background_scripts.js.
Update background.html to load the Image Loader as JS modules.
Optimize_webui to run Image Loader as JS modules.

Tests:
Load image loader and check that there is no additional error in
the console.
Current integration tests pass.

Bug: 1133186
Change-Id: I3cdf9fd2a59bca080b15c3efad51739a15fa93e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2635384
Commit-Queue: Jeremie Boulic <jboulic@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844703}