Prevent unnecessary duplicate inserts of code

[photodow]

I've done a lot to clean this up already where the code just kept getting injected every time the page was validated, but there may be still some work still around this particular as it relates to the validation step. It's not a huge issue at the moment, but something I want to keep an eye on and fix as soon as I get a chance.

We've used declarativeContent before, but could it be used to continue validating a page after it's been injected?https://developer.chrome.com/extensions/declarativeContent

https://github.com/carbon-design-system/devtools/blob/master/App/background/tasks/validatePage.js#L17-L21