Issue full paint invalidation when fragment count changes.

Only do this if a fragment count of greater than 1 is involved, though.

We cannot re-use the cached drawing if a box gets refragmented. We would
otherwise do that if the total block-size of the box remains unchanged,
and its offset doesn't change.

Note that the call to FragmentCountDidChange() when *adding* fragments
isn't strictly required, since we'd detect a geometry change when new
FragmentData entries are added, but it doesn't seem right to rely on
that.

We now need to be more careful about changing the size of the
layout_results_ vector in LayoutBox, as that will trigger repaint.
Therefore, only shrink the vector when we have reached the last
fragment, which means that if the final fragment count turns out to
remain unchanged, we'll leave the vector size alone during layout, which
should be a good thing anyway.

Bug: 1285795
Change-Id: I8a0eaa047bb448dacb2f0325f94fb361976ae7de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3393649
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/main@{#960169}