[CSS Grid Layout] Support sparse in auto-placement algorithm

[CSS Grid Layout] Support sparse in auto-placement algorithm
https://bugs.webkit.org/show_bug.cgi?id=134544

Reviewed by Sergio Villar Senin.

Source/WebCore:
This patch implements sparse mode for auto-placement algorithm, which is
the default mode in the new grid-auto-flow syntax. It keeps track of the
auto-placement cursor in
RenderGrid::placeAutoMajorAxisItemsOnGrid() and updates it accordingly
when auto-positioned items are placed.
If we're in dense mode it resets the cursor after each item (which keeps
the old behavior that was using dense mode by default).

GridIterator has been adapted to look for empty areas from a given
position in both directions.

Test: fast/css-grid-layout/grid-auto-flow-sparse.html

* rendering/RenderGrid.cpp:
(WebCore::RenderGrid::GridIterator::GridIterator): Modify constructor to
add an optional argument for the varying index. This allows to look for
empty areas in both axis.
(WebCore::RenderGrid::placeAutoMajorAxisItemsOnGrid): Defined the
auto-placement cursor and rested after each item if we're in dense mode.
(WebCore::RenderGrid::placeAutoMajorAxisItemOnGrid): Use auto-placement
cursor to look for empty areas from the last auto-positioned item
placed.
* rendering/RenderGrid.h: Modify placeAutoMajorAxisItemOnGrid() header
to receive the auto-placement cursor.

LayoutTests:
Test cases have been adapted accordingly, adding new cases to cover both
sparse and dense options.

* fast/css-grid-layout/grid-auto-flow-resolution-expected.txt:
* fast/css-grid-layout/grid-auto-flow-resolution.html:
* fast/css-grid-layout/grid-auto-flow-sparse-expected.txt: Added.
* fast/css-grid-layout/grid-auto-flow-sparse.html: Added.
* fast/css-grid-layout/grid-container-change-named-grid-lines-recompute-child.html:
* fast/css-grid-layout/grid-item-auto-placement-automatic-span-expected.txt:
* fast/css-grid-layout/grid-item-auto-placement-automatic-span.html:
* fast/css-grid-layout/grid-item-auto-placement-definite-span-expected.txt:
* fast/css-grid-layout/grid-item-auto-placement-definite-span.html:
* fast/css-grid-layout/grid-item-removal-auto-placement-update.html:
* fast/css-grid-layout/named-grid-lines-with-named-grid-areas-resolution.html:
* fast/css-grid-layout/resources/grid.css:
(.autoRowAutoColumnSpanning2):
(.autoRowSpanning2AutoColumn):
(.autoRowSpanning2AutoColumnSpanning3):
(.autoRowSpanning3AutoColumnSpanning2):
(.gridAutoFlowColumnDense):
(.gridAutoFlowRowDense):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@171082 268f45cc-cd09-0410-ab3c-d52691b4dbfc