Feature request: DevTools also queries ComputedStyle::isStackingContext() from the element. When z-index is auto while isStackingContext is true, display z-index as "z-index: 0 (forced)".

Some background:
Many of CSS properties will force an element to create a stacking context, for example, opacity other than 1, transforms, filters, etc... Traditionally we adjust z-index:auto to z-index:0 in StyleAdjuster, but we no longer do that since 4 month ago: https://codereview.chromium.org/2047283002

Since that CL the inspector no longer shows the used value of z-index, but the computed value (prior to adjustment), which I think is more correct. However the used value is extremely valuable in debugging some CSS corner cases.