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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/+/1748949Reviewed-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}