Reset the box model to content-box in the web debug toolbar

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

This makes the styles compatible with CSS frameworks changing the box model to box-sizing using a universal selector (Bootstrap 3 for instance).