Do less in perf test tall-content-short-columns.html

Change the test so that it can do about one run per second, instead of
taking forever (I waited about a minute without any output, so I gave
up). Also remove the short piece of text inside the tall block, to keep
it pure and simple.

This test is useful for measuring performance scalability issues when
many fragmentainers are involved, but it became way too slow with
LayoutNGBlockFragmentation. It is acceptable that NG block frag is
slower at such tests, since we now actually have to perform some layout
per fragmentainer. But we can do better. There are multiple separate
O(n^2) issues, both in layout, pre-paint and paint.

There will be CLs coming up, that will improve the speed of this test
drastically.

Change-Id: Ieeb3662fd06651e466c9bb0ebca8fbbc5a719c4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4819186
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1189192}