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

Sparse mode for auto-placement algorithm was not implemented yet.

This patch implements sparse mode for auto-placement algorithm. 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.

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

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

TEST=fast/css-grid-layout/grid-auto-flow-sparse.html

BUG=384099

Review URL: https://codereview.chromium.org/362733002

git-svn-id: svn://svn.chromium.org/blink/trunk@178262 bbb929c8-8fbe-4397-9dbb-9b2b20218538

1 files changed, 1 insertions, 1 deletions

diff --git a/third_party/WebKit/LayoutTests/fast/css-grid-layout/grid-container-change-named-grid-lines-recompute-child.html b/third_party/WebKit/LayoutTests/fast/css-grid-layout/grid-container-change-named-grid-lines-recompute-child.html
index 1e0a755..b9840f2 100644
--- a/third_party/WebKit/LayoutTests/fast/css-grid-layout/grid-container-change-named-grid-lines-recompute-child.html
+++ b/third_party/WebKit/LayoutTests/fast/css-grid-layout/grid-container-change-named-grid-lines-recompute-child.html
@@ -4,7 +4,7 @@
 <script src="../../resources/check-layout.js"></script>
 <style>
 .grid {
-    grid-auto-flow: row;
+    grid-auto-flow: row dense;
 }
 #firstGridItem {
     grid-row: auto;