Revert of Files.app: Split background.js (patchset #4 id:60001 of https://codereview.chromium.org/562103002/)

Reason for revert:
This patch breaks the test.

Original issue's description:
> Files.app: Split background.js
>
> This patch splits the background.js into the 3 files (background_base.js, app_window_wrapper.js, background.js), extracting the common classes.
>
> This patch itself doesn't change any functionality, but the extracted files will be used in the separated audio player app in near future.
>
> BUG=375039
> TEST=browser_test passes
>
> Committed: https://crrev.com/479878c12c436c0bec667483df29fe1f23b710a2
> Cr-Commit-Position: refs/heads/master@{#294998}

[email protected]
NOTREECHECKS=true
NOTRY=true
BUG=375039

Review URL: https://codereview.chromium.org/563743004

Cr-Commit-Position: refs/heads/master@{#295019}

Author: yoshikig