core(screenshot-thumbnails): improve caching

[midzer]

Summary

Improve caching logic for already existing thumbnail data to skip unnecessary (expensive) function calls

Should result in better performance on complete LH run. Did not measure though :|

More performance without overengineering is a good thing :)

Related Issues/PRs

#5957

[brendankenny]

nice, thanks for the fix!

[paulirish]

I tried measuring this (quickly). Nothin too substantial, but it may be going from 230ms to 215ms on my test trace. Heh, so not as much of a win as we expected.

oh well, the code looks a lot better now. :)