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This is a reland of b3373919

Original change's description:
> Convert ui/ARIAUtils.js to an ES module
>
> ES modules allow files to explicitly import and export symbols [1].
> This patch migrates ARIAUtils.js to an ES module, while maintaining
> backwards compatibility with existing usages of `window.UI.ARIAUtils`.
>
> The build system is updated to copy these modules verbatim, as it no
> longer require pre-processing to be loaded. The `module.json` no longer
> includes this file as a script, to make sure it is not double-bundled
> into `shell.js`. Instead, it is added to the new field `modules`, which
> contains all modules. By doing so, it becomes clear which files are
> converted to modules already and which require the legacy references
> to the global scope.
>
> This module and future modules will be imported from `root.js`, which is
> added to all existing HTML application files.
>
> Design Doc: https://docs.google.com/document/d/1h9dOy3nNPNfZ2AtZXzB-DwJqG4Oo37WWvKLCuzCcPzo/edit?usp=sharing
>
> [1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import
>
> TBR=jochen@chromium.org
>
> Change-Id: Ie79c8f2fce3aff96fa28af0b575eae39bfe8e1a5
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1748949
> Reviewed-by: Jochen Eisinger <jochen@chromium.org>
> Reviewed-by: Yang Guo <yangguo@chromium.org>
> Commit-Queue: Tim van der Lippe <tvanderlippe@google.com>
> Cr-Commit-Position: refs/heads/master@{#697238}

Change-Id: I7544cbf4364d0b1a3c2f137a4d17a1afdb1d4aed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1810534Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Tim Van der Lippe <tvanderlippe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697626}