Bug 264148

When inspecting an element with a style applied from an `adoptedStyleSheet`, the inspector displays the style as originating from a locked `User Agent Style Sheet`. This makes it impossible to use the inspector to alter the style or know from where it's originating.

See https://jsbin.com/juhucaw/2/edit?html,output.

Steps to reproduce:
1. Inspect the blue div
2. Look at it's styles.

Expected:
1. styles are editable in the inspector
2. styles are reported as coming from an adoptedStyleSheet

Actual:
1. styles are locked in the inspector
2. styles are reporeted as coming from `User Agent Style Sheet`.