[Performance] Convert ASLayoutElementSize to atomic. #trivial

[appleguy]

ASLayoutElementStyle is one of the most frequently accessed objects in the framework. Sometimes it's also used on the main thread, further increasing the value of performance optimizations.

We saw very significant wins once other properties on this object were converted to atomics. A profile I just took suggests that there's meaningful opportunity in converting ASLayoutElementSize as well, of about 2% total CPU.