misc: tweaks for devtools client

[paulirish]

1. match score scale order to LS. (green, orange, red)
2. negative margin of metrics is now scaled against circle-size, which is the intent.
3. expose lh-background methods appropriately in a worker

[brendankenny]

hmm, are you seeing the same runLighthouseInExtension is not defined error that's in travis when you run yarn test-extension? I wouldn't think it would be affected by that change to lighthouse-background.js

[brendankenny]

ok, yes, I see the error locally too

[brendankenny]

LGTM