Fetch CPU speed limit asynchronously in ThermalStateObserverMac

The IOPMCopyCPUPowerStatus function, used to get the CPU speed limit, is
a potentially blocking IOKit call. Currently, ThermalStateObserverMac
calls this synchronously on a sequence that may run on the main thread,
which can contribute to UI jank and increase PPM.

To mitigate this, this CL introduces an asynchronous path for fetching
the speed limit, moving the blocking call to a background thread using
PostTaskAndReplyWithResult.

This new asynchronous behaviour is gated by the existing
base::features::kReducePPMs feature flag. This change is modeled on the
asynchronous pattern found in BatteryStatusManager.

Bug: 424432184

Change-Id: I5505fca411b17d8eff3b25654988729a9d548ac8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6652349
Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Commit-Queue: Alex Attar <aattar@google.com>
Cr-Commit-Position: refs/heads/main@{#1476153}