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idle_time_user_hz is measured in USER_HZ (clock ticks), a system
constant accessible via sysconf(_SC_CLK_TCK) (clock ticks / second).
Dividing the two results in the number of idle seconds and multiplying
by 1000 gets the result in milliseconds.

Add a wrapper around LogicalCpuInfoPtr::UncheckedConvertPtr to make
unittesting of the function easier without mocking the system call
sysconf().

Add tests for checking LogicalCpuInfoPtr::UncheckedConvertPtr.

Bug: b:161219494
Change-Id: I0c01f63100beea5315e1cc8f215eeb2fac6e76ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2301144Reviewed-by: Oleh Lamzin <lamzin@google.com>
Reviewed-by: Mahmoud Gawad <mgawad@google.com>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Laurențiu Olteanu <lolteanu@google.com>
Cr-Commit-Position: refs/heads/master@{#790293}