[CSS Grid Layout] RenderGrid methods should take RenderBox references
https://bugs.webkit.org/show_bug.cgi?id=136630

Reviewed by Darin Adler.

RenderGrid has several methods that take pointers to RenderBox
instances but the body of those methods assumes that the objects
will never be NULL. We should replace them by references.

* rendering/RenderGrid.cpp:
(WebCore::RenderGrid::computeUsedBreadthOfGridTracks):
(WebCore::RenderGrid::logicalContentHeightForChild):
(WebCore::RenderGrid::minContentForChild):
(WebCore::RenderGrid::maxContentForChild):
(WebCore::RenderGrid::resolveContentBasedTrackSizingFunctions):
(WebCore::RenderGrid::resolveContentBasedTrackSizingFunctionsForItems):
(WebCore::RenderGrid::insertItemIntoGrid):
(WebCore::RenderGrid::placeItemsOnGrid):
(WebCore::RenderGrid::createEmptyGridAreaAtSpecifiedPositionsOutsideGrid):
(WebCore::RenderGrid::placeSpecifiedMajorAxisItemsOnGrid):
(WebCore::RenderGrid::placeAutoMajorAxisItemsOnGrid):
(WebCore::RenderGrid::placeAutoMajorAxisItemOnGrid):
(WebCore::RenderGrid::layoutGridItems):
(WebCore::RenderGrid::cachedGridCoordinate):
(WebCore::RenderGrid::gridAreaBreadthForChild):
(WebCore::RenderGrid::findChildLogicalPosition):
* rendering/RenderGrid.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@173427 268f45cc-cd09-0410-ab3c-d52691b4dbfc