ResizeObserver-based fitting observer

[yhatt]

Currently we use polling-based observation for fitting header. It tracks the change of size includes CSS animation certainly, but it has a big effect to the performance.

Chrome >= 64 is supported ResizeObserver to observe change the size of element with better performance. It has better to use it if possible.

[yhatt]

Safari 13.1 had shipped at this month, and now all modern browsers have implemented ResizeObserver. We may start working for new implementation of auto scaling.

If completed, the current polling-based logic would only use as fallback.