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Document which are singletons vs. per-profile.

Collapse all the chromeos stuff into one block.

Bug: none
Change-Id: I869c1ad4b1dd889de017743571b41f3ce31affb3
Reviewed-on: https://chromium-review.googlesource.com/1101829Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567470}