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JS modules for:
- //ui/file_manager/file_manager/background/js/
  - media_scanner.js
  - mock_media_scanner.js
  - media_scanner_unittest.m.js
- //u/file_manager/externs/background/media_scanner.js

Because Closure doesn't deal well importing the same namespace from
different files, I started splitting the "importer" namespace so each
file will have its own namespace.  Files in the "externs" directory will
have the suffix "Interface" on their namespace.  I'll use this logic for
all files related to "importer" namespace.

Bug: 1133186
Change-Id: I55b14aaa58a140e1e754ceaf15c51219df19578a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2596281Reviewed-by: Jeremie Boulic <jboulic@chromium.org>
Commit-Queue: Jeremie Boulic <jboulic@chromium.org>
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837991}