So this mean's that we'll put this information into the imageloader.json file? In that case we need a plan to deal with components that don't have this information. 

We'll need to bump the manifest version, or add code to parse out an optional field, which is probably fine.

Yeah, we make it an optional field in the json file. Something like this: https://chromium-review.googlesource.com/c/508441/

Just wondering, what is this being used for?

This is for making component compatible with OS (since component is released independent of OS). 

(go/cros_component_compatibility)

So the environment version number is a Chrome OS platform version number? Why not call it that instead of introducing a new term?

actually environment version number is per component. It up-revs only when environment changes cause incompatibilities of that component. 

Not long needed.