[CSS Grid Layout] Run the content-sized tracks sizing algorithm only when required

[CSS Grid Layout] Run the content-sized tracks sizing algorithm only when required
https://bugs.webkit.org/show_bug.cgi?id=124039

Reviewed by Dean Jackson.

PerformanceTests:

From Blink r156028 and r156168 by <jchaffraix@chromium.org>.

New performance tests for layouts in grids with fixed size tracks.

* Layout/fixed-grid-lots-of-data.html: Added.

Source/WebCore:

The current code runs the content sized track sizing algorithm all
the time, which forces a layout even when the track is not
content-sized. This change improves the situation by applying two
optimizations. In the first one, we bail out the algorithm if we
detect that we don't need to run it. And by the second one we
reduce the amount of recomputations by only iterating over the
content sized tracks instead of all of them. Both changes follow
the ideas introduced in Blink r156028 and r156168 by
<jchaffraix@chromium.org>.

As we changed the way we iterate over children (we use the
GridIterator now) the way they're stored in the RenderGrid changes
too. If a item spans through several "cells" inside the grid, we
will have a reference to it on each of them.

These two changes account for a ~3200% improvement on a i7 M620 in
the test that accompanies this change (15.5 vs 520 run/s).

New perf test: PerformanceTests/Layout/fixed-grid-lots-of-data.html

* rendering/RenderGrid.cpp:
(WebCore::RenderGrid::computedUsedBreadthOfGridTracks): Keep track
of content sized tracks and only iterate over them.
(WebCore::RenderGrid::resolveContentBasedTrackSizingFunctions):
(WebCore::RenderGrid::resolveContentBasedTrackSizingFunctionsForItems):
Early return if there are no tracks to pass to the algorithm.
* rendering/RenderGrid.h:
* rendering/style/GridLength.h:
(WebCore::GridLength::isContentSized):
* rendering/style/GridTrackSize.h:
(WebCore::GridTrackSize::isContentSized):

LayoutTests:

From Blink r156028 and r156168 by <jchaffraix@chromium.org>.

Subtle baseline change due to grids triggering less layouts, which
changes the frame rects between 2 subsequent layouts thus changing
the repaint rectangles.

* fast/css-grid-layout/grid-item-change-column-repaint-expected.txt:
* fast/css-grid-layout/grid-item-change-row-repaint-expected.txt:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@159684 268f45cc-cd09-0410-ab3c-d52691b4dbfc