Lightbox performance review

[cathyxz]

1. Go through Chrome devtools performance profiling to identify bottlenecks.
2. Go through all measurements and mutations and make sure that they only happen within vsync blocks.
3. Review the component design to see if there are other performance optimizations that we can do.

[cathyxz]

Results of perf profiling on Chrome Devtools

Here's a rough list of perf bottlenecks on open lightbox:

• getBoundingClientRect
• animation add
• Gestures
• updateDescriptionBox (the use of innerText)?

on change slide:

• shouldTriggerClick
• updateImageSrc

[cathyxz]

We did a couple of passes on this via #13809, #14141, and #13679. Performance on low-end phones is now decent after #14119, with the caveat of needing to not use 5MB+ images.