Reduce paint when dragging overview item.

Optimization. Should have an imperceptible visual impact. All components
of an overview item were being re-painted unnecessarily as the item
was being dragged. This eliminates all painting except for one
known case: When the window's label is long and gets truncated as the
overview item shrinks, the label gets re-painted.

Bug: 371324817
Change-Id: I416440608cdd22be8b81bdd6138dafbc94a483de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6008793
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Eric Sum <esum@google.com>
Cr-Commit-Position: refs/heads/main@{#1394470}