feat(DummyInputs): Optimizing the dummy transform offsets to reduce the reflows.

[mshoho]

getComputedStyle() causes reflow, accessing element.scrollTop and element.scrollLeft causes reflow. Setting element.style.top and element.style.left in between the above makes it all even worse.

This PR:

1. Caches the result of the things that cause reflow for the bulk during the update.
2. Splits computing the dummy input position and setting it into two phases to do the bulk computation first and the bulk position update next.
3. Improves the data structures usage in the computation.

[ling1726]

How do you know these changes improve perf? have you taken any profiles or before vs after?

[mshoho]

How do you know these changes improve perf? have you taken any profiles or before vs after?

We are looking at the snapshots with the perf team. This one is definitely improving it and I've just pushed another update to improve the timers (after testing the previous change with the perf folks). Releasing next canary right now and once I have the confirmation it's making things even better, it should be ready to be merged.