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This CL uses the Hierarchy infrastructure added in [1] to register
subpages and settings for each section. It also changes
OsSettingsSection::RegisterHierarchy() to a pure virtual function so
that all sections must implement this functionality.

Additionally, this CL adds a test which verifies that metadata is
registered for every subpage/setting. This ensures that as new content
is added to the settings page, it must be registered to pass the CQ.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2203149

Bug: 1080892
Change-Id: Ia9736f3f841c1c61202e1fe59bdb3793f0874bf2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2204440Reviewed-by: Regan Hsu <hsuregan@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Auto-Submit: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#769496}