So I've been doing a quick check.

If I use a small grid 100px by 100px, the local_visual_rect is 800x600.

However if I use a big grid 2000px by 2000px, the rect is 2000x2000.
If I go bigger like 10000px by 100000px, the rect is 4800x4600.
So only in this last case our optimization is doing some useful work.

So I have been doing some tests under perf_tests/paint.
I wrote a test of a grid of 500 rows and 500 columns and one item per cell,
where each column and row has a breath of 5px (the total size is 2500px x 2500px).
In this case the optimization is not useful at all, as the CullRect is 4800x4600
and we're going to paint all the items anyway,
actually the "optimization" is adding an overhead.
With current code the test takes an average of 5239ms
while removing the optimization it takes an average of 4439ms.

I also checked a big grid of 500x500 with breaths of 20px (size 10000 px x 10000 px).
In this case the optimization should be saving us from paining a lot of items
instead of painting 250000 items we'll be painting only ~62500 items.
Here the results of current code were almost the same than without the optimization,
an average of 1730ms and without the optimization 1737ms.

So I believe we should get rid of the "optimization"
that is no longer doing anything useful and simplify our code.

For future reference, there's a thread in blink-dev about this topic:
https://groups.google.com/a/chromium.org/d/msgid/blink-dev/407a210a-cb90-7817-da48-62b532fb821e%40igalia.com

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/71265f961b86780b8abd297295f2b9b936f4ac59

commit 71265f961b86780b8abd297295f2b9b936f4ac59
Author: Manuel Rego Casasnovas <rego@igalia.com>
Date: Fri Sep 21 08:34:48 2018

[css-grid] Remove painting "optimization"

Since a while ago the CullRect we get in GridPainter::PaintChildren()
is really big matching the size of the grid
if it's smaller than 4800x4600 pixels.

For that reason the painting optimization we have
is not doing anything useful if the grid is smaller than that size,
as all the grid items are painted anyway.
Even it's adding some overhead in some cases making things slower.

For that reason we're getting rid of the optimization
which allows us to simplify the grid layout code
and rely on more common painting code.

No new test as it's covered by existent ones.

BUG= 853654 

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ida3cf7cc081b8671ea10a300891b0825f76258bd
Reviewed-on: https://chromium-review.googlesource.com/1235998
Reviewed-by: Sergio Villar <svillar@igalia.com>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Manuel Rego <rego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#593120}
[modify] https://crrev.com/71265f961b86780b8abd297295f2b9b936f4ac59/third_party/blink/renderer/core/layout/layout_flexible_box.cc
[modify] https://crrev.com/71265f961b86780b8abd297295f2b9b936f4ac59/third_party/blink/renderer/core/layout/layout_grid.cc
[modify] https://crrev.com/71265f961b86780b8abd297295f2b9b936f4ac59/third_party/blink/renderer/core/layout/layout_grid.h
[modify] https://crrev.com/71265f961b86780b8abd297295f2b9b936f4ac59/third_party/blink/renderer/core/paint/BUILD.gn
[modify] https://crrev.com/71265f961b86780b8abd297295f2b9b936f4ac59/third_party/blink/renderer/core/paint/block_painter.cc
[modify] https://crrev.com/71265f961b86780b8abd297295f2b9b936f4ac59/third_party/blink/renderer/core/paint/block_painter.h
[delete] https://crrev.com/a047f6f2084cffef50a8ce143babb9934f864c7b/third_party/blink/renderer/core/paint/grid_painter.cc
[delete] https://crrev.com/a047f6f2084cffef50a8ce143babb9934f864c7b/third_party/blink/renderer/core/paint/grid_painter.h