UWP doesn't allow reading regkeys (#594)

* 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.

absl/base/internal/sysinfo.cc

@@ -72,6 +72,12 @@ static int GetNumCPUs() {
 #if defined(_WIN32)
 
 static double GetNominalCPUFrequency() {
+// UWP apps don't have access to the registry and currently don't provide an
+// API informing about CPU nominal frequency.
+#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP) && \
+    !WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP)
+  return 1.0;
+#else
 #pragma comment(lib, "advapi32.lib")  // For Reg* functions.
   HKEY key;
   // Use the Reg* functions rather than the SH functions because shlwapi.dll
@@ -91,6 +97,7 @@ static double GetNominalCPUFrequency() {
     }
   }
   return 1.0;
+#endif // WINAPI_PARTITION_APP && !WINAPI_PARTITION_DESKTOP
 }
 
 #elif defined(CTL_HW) && defined(HW_CPU_FREQ)