Add tooltip for next/prev month arrows for calendar popup

Prior to this change, the next and previous month arrows for calendar
popup did not have a tooltip. With this CL, we add a tooltip where the
tooltip title matches the arrows' aria-label (e.g. "Show previous month"
and "Show next month").

Changes: https://imgur.com/a/fjE6p49

Bug: None
Change-Id: I2649a84b0eb3d97679e8db13e3ab4ed7ccb546ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4779473
Reviewed-by: Mason Freed <masonf@chromium.org>
Commit-Queue: Stephanie Zhang <stephanie.zhang@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1183939}