[CSS Grid Layout] Avoid mark grid as dirty always when removing a child

Source/core/rendering/RenderGrid.cpp

Index: Source/core/rendering/RenderGrid.cpp
diff --git a/Source/core/rendering/RenderGrid.cpp b/Source/core/rendering/RenderGrid.cpp
index fce09911d7c73dce2f2a867a7c8c1123b7a63e20..b54cd4bb62fd14bfa97bc82ff1ede5ad585f81d1 100644
--- a/Source/core/rendering/RenderGrid.cpp
+++ b/Source/core/rendering/RenderGrid.cpp
@@ -223,8 +223,24 @@ void RenderGrid::removeChild(RenderObject* child)
         return;
 
     ASSERT(child->isBox());
-    // FIXME: We could avoid dirtying the grid in some cases (e.g. if it's an explicitly positioned element).
-    dirtyGrid();
+
+    if (style()->gridAutoFlow() != AutoFlowNone) {

[Julien - ping for review, 2014/03/31 18:36:51]

Note that if you don't have any grid item that are auto positioned you could
avoid dirtying the grid too. Probably worth a comment / FIXME but it seems to me
that it's a corner case that we shouldn't care about (for now).

+        // The grid needs to be recomputed as it might contain auto-placed items that will change their position.
+        dirtyGrid();
+        return;
+    }
+
+    const RenderBox* childBox = toRenderBox(child);
+    GridCoordinate coordinate = m_gridItemCoordinate.get(childBox);

[Julien - ping for review, 2014/03/31 18:36:51]

You could avoid the 2 hash lookup by using m_gridItemCoordinate.take(childBox)
(or find() but the code will be a bit less nice).

+
+    for (size_t row = coordinate.rows.initialPositionIndex; row <= coordinate.rows.finalPositionIndex; ++row) {
+        for (size_t column = coordinate.columns.initialPositionIndex; column <= coordinate.columns.finalPositionIndex; ++column) {
+            GridCell& cell = m_grid[row][column];
+            cell.remove(cell.find(childBox));
+        }
+    }
+
+    m_gridItemCoordinate.remove(childBox);
 }
 
 void RenderGrid::styleDidChange(StyleDifference diff, const RenderStyle* oldStyle)