* UWP doesn't allow reading regkeys.
Unfortunately, UWP also doesn't offer an API for returning nominal processor frequency at this moment. Other options would require apps depending on abseil-cpp to be packaged with extra manifest data or libraries for bridging platforms.
This change pushes the unsupported APIs accessing the registry behind a define guard. This define guard makes GetNominalCPUFrequency to compile and run as usual on desktop, but it will return the value 1.0 on UWP Apps (Store).
* Using WINAPI_FAMILY_PARTITION family of macros for detecting when building for
UWP or Desktop.
* Simplifying comment to please the lint tool.
