Allows the use of PageUp and PageDown in sequence and schedule phases.

Since graphmultiview had a tabindex of 0, it was sitting on top of
sequence and schedule. This blocked the use of PageUp and PageDown
to scroll in these phases.

Bug: v8:7327
Change-Id: I4fc129cd9d5ea82e469cd4b67c12a455ec920317
Notry: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1914207Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64947}