具有的系统内存容量（只读）。
此为系统内存容量的近似值（以兆字节为单位）。
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.
另请参阅：SystemInfo.graphicsMemorySize。