[CSS Grid Layout] Fix painting avoid repaint items and respect DOM ordering

After the performance optimizations some issues appear painting the grid:
* Grid items are painted several times if they span.
* Global DOM ordering is not respected. Before this patch ordering was
  only respected inside every grid cell.

The patch fixes these issues keeping a map with the information about
the index position of the items in the grid. The map is used in the
painting method to generate a Vector that is properly sorted using that
information. Thanks to this changes painting code in RenderGrid has been
simplified.

The changes introduced in the past in OrderIterator to have a iterate
over a subset of the items have been removed as they were not needed
anymore.

Checking painting performance on a 200x200 grid it is in the same
numbers than before this patch.

BUG=248151,256804
TEST=LayoutTests/fast/css-grid-layout/grid-painting-items-only-once.html
TEST=LayoutTests/fast/css-grid-layout/grid-painting-respect-dom-order.html

Review URL: https://codereview.chromium.org/181003007

git-svn-id: svn://svn.chromium.org/blink/trunk@174490 bbb929c8-8fbe-4397-9dbb-9b2b20218538