WorkerNavigator: deviceMemory property

The deviceMemory read-only property of the WorkerNavigator interface returns the approximate amount of device memory in gigabytes.

The reported value is imprecise to curtail fingerprinting. It's approximated by rounding down to the nearest power of 2, then dividing that number by 1024. It is then clamped within lower and upper bounds to protect the privacy of owners of very low-memory or high-memory devices.