coral: calculate extra number for icon label

Currently, coral chip would get all tab and app icons even we only need
at most 4.  Also, the extra number is hard coded.

The cl limits the number of requested icons such that when the total
number of tabs and apps exceeds 4, we only get 3 icons. The extra number
will also be calculated before running the barrier callback.

Bug: b:369932586
Change-Id: I3c66dd84eee4259817e2fea89844eb0d7ca4e05b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5896049
Reviewed-by: Matthew Mourgos <mmourgos@chromium.org>
Commit-Queue: Xiaodan Zhu <zxdan@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1362044}