Drive-by: Fix z-index (stacking) issue with hovers and pane
resize bars.

Change-Id: Ibd028a666681face1ccd6ec36b388f1ef25b1393
Notry: true
Bug: v8:7327
Reviewed-on: https://chromium-review.googlesource.com/c/1396420
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58573}