MHz単位のプロセッサー周波数（読み取り専用）
This function returns the nominal CPU frequency in MHz, or zero if unknown. Note that the actual CPU frequency might vary
depending on current load and power conditions, especially on low-powered devices like phones and laptops.
On some platforms it's not possible to query the CPU frequency. Currently such platforms are iOS and WebGL.
See Also: SystemInfo.processorCount, SystemInfo.processorType.