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Add the build rules to build the background page as JS module.

Bug: 1133186
Change-Id: Ic2759ebd3475ca01f49f153020a16a99673a4fc8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537448
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Jeremie Boulic <jboulic@chromium.org>
Commit-Queue: Jeremie Boulic <jboulic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827972}