The OverlayAgent tests that will land in https://chromium-review.googlesource.com/c/chromium/src/+/1283551 make the assumption that the highlighting layer is a child of the root view. This is correct for Aura platforms, but will not be correct when the Mac implementation lands.

Instead, since there's no "root window" concept on Mac, the layer will be parented to the appropriate widget. This bug is a reminder to fix this so that test coverage isn't lost.