We've been having a discussion about this API and the privacy
implications on the dev-webapi mailing list. The thread is entitled
"Should the idle service be exposed to the unprivleged web?"
Although we have not reached complete consensus in that thread, a
consensus has formed that we will not expose device-idle to the
unprivileged web. (We do currently; the consensus does not match our
current behavior.) We may expose a page-idle value to the
unprivileged web, but that will not expose any information that's not
already available to a page, so therefore it's not a privacy concern.
Given that we have consensus to not expose privacy-sensitive
information to the unprivileged web, I don't think a privacy review of
this feature is necessary at this time. But in any case, please
direct those interested in the privacy discussion to that thread, so
we don't have to repeat our comments in a separate privacy review call
for comments procedure.