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This CL splits ExternalWebAppManager's config loading flow into discrete
steps and removes the various testing entry points that omit or
duplicate logic used by the real thing.

Tests can now inject data (file_utils, config_dir, configs) into the
config loading pipeline instead of having separate mini pipelines just
for them.

This change is in preparation for adding a new "FilterExtensionIds" step
to the config loading flow. Without this refactor there would be
duplication of logic between real and test code code paths that would be
prone to bitrot and missing real coverage.

Bug: 1128801
Change-Id: Ie5e87b26ab47782fab6589a4127df3ea5e876fec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2423823Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Glen Robertson <glenrob@chromium.org>
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810025}