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The ThermalResource reports kOveruse or kUnderuse every 10 seconds
while it has a registered listener and the DeviceThermalMeasurement is
known. Because OnThermalMeasurement() only happens when the thermal
state changes, repeated kOveruse is needed to adapt multiple steps.

Based on [1] and manual observations, we do not want to adapt if
thermals are kNominal or kFair so we map these to kUnderuse. But if
thermals are kSerious or kCritical this is a strong signal from the OS
that "corrective action" or "immediate corrective action" is needed.

It can easily take a minute before the thermal state changes after load
distribution has changed, so the effects of ThermalResource is likely to
result in either maximally adapted or not adapted at all. The repeated
interval of 10 seconds was somewhat arbitrarily chosen but was chosen as
a tradeoff between giving the OS time to measure the new load and not
making the resource too spammy.

This CL implements and tests the resource, but does not wire it up to
the webrtc::PeerConnection or
RTCPeerConnectionHandler::OnThermalMeasurement(). This will be done in a
separate CL.

[1] https://developer.apple.com/library/archive/documentation/Performance/Conceptual/power_efficiency_guidelines_osx/RespondToThermalStateChanges.html

Bug: chromium:1094844
Change-Id: I7dee0842ba786babbacb7e9d19acb58a1c9daa05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2247769Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Mirko Bonadei <mbonadei@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780189}