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This CL:
* Wraps each settings subpage with a dom-if
* Adds a new SettingsPageVisibility behavior that allows tests
  to control page visibility.
* NOTE: This changes the page load order (subpages are all loaded
  after the main pages and their dependencies). This has the side
  benefit of making Prefs more likely to load before the pages do.
* Fixes an inconsistency with the naming of the OnStartup section

NOTE: This does not impact i18n_template.js since that is loaded after <cr-settings> in settings.html, however if we enable a subpage after loading (e.g. in a test) then i18n_template.js will not be processed and would need to be run explicitly to include translations.

BUG=560432
R=dbeam@chromium.org, michaelpg@chromium.org

Review URL: https://codereview.chromium.org/1477773003 .

Cr-Commit-Position: refs/heads/master@{#364470}