don't calculate pixel width of input

[tinovyatkin]

I'm probably missing something, as there is no visual test for this functionality, but.
You currently have a great deal of code to calculate exact pixel width of helper INPUT element and it's doing good job. However, I think that almost the same functionality with little bit better compatibility with accessibility displays and other rare cases, as well as with much less code can be obtained by using well supported ch unit.
ch is a width of 0 in current font and it's not exact measurements of value string width, but you are using pre and magic number of 75% in this function anyway, so this variant can be tuned in similar way (something like length * 0.75 should be pixel perfect) if you want. For my taste, I will leave it like this.

Also found that current tests are using non-existent _placeholderValue property, but still passing well 🧐

Unfortunately, this functionality can't be tested with JSDOM due to lack of ch support there (I've filled appropriate Pull Request), so, I've tested by temporary using em units, and also manually in browser.

Now, it's up to you, as it's not pixel perfect and have some edge case where it leaves a little of space at the end on some strings. But I ❤️ this solution.

[tinovyatkin]

My Pull Request to JSDOM/cssstyle was merged tonight, so, soon it will be also testeable feature (with next JSDOM release hopefully).

[jshjohnson]

This is awesome 👌 thanks