[css-writing-modes] The interaction between display:contents and unicode-bidi should be clearly defined and tested

The interaction between the unicode-bidi property and display: contents should probably be defined more clearly than it is today, and also tested. In at least Chrome, at least some values of unicode-bidi work on elements that are display: contents. We should both decide what the desired behavior is and make sure there are tests for it. (There may be some behaviors that are clearly defined today because of explicit wording in the spec that was written before display: contents existed, but those definitions may not really be what we want.)

(This came from a review comment I made on a Chromium CL.)

Start with the CSS Writing Modes Level 4 unicode-bidi section and the CSS Display Level 3 display: contents definition, then review the linked Chromium review comment for the motivating behavior. Determine the intended interaction, document the agreed specification wording, and add tests covering the relevant unicode-bidi values on display: contents elements.