システムメモリの量（読み取り専用）
システムメモリのおおよその容量（メガバイト単位）です。
WebGL: This function will return the current size of the asm.js/WebAssembly heap. Note that when using WebAssembly, the heap size can increase at run-time if the Unity content memory-usage exceeds the initial memory size.
Windows Store Apps: This function is not supported on Windows Store Apps and will always return 0.
Additional resources: SystemInfo.graphicsMemorySize.