Initial implementation of LayoutNG's block layout algorithm for floats.

third_party/WebKit/Source/core/layout/ng/ng_layout_opportunity_iterator.cc

Index: third_party/WebKit/Source/core/layout/ng/ng_layout_opportunity_iterator.cc
diff --git a/third_party/WebKit/Source/core/layout/ng/ng_layout_opportunity_iterator.cc b/third_party/WebKit/Source/core/layout/ng/ng_layout_opportunity_iterator.cc
index 76977630c2a784e71e646b72bf7d09017d55fd16..65e8af83fddaef126ecb06402dfd3985b7ddbcb0 100644
--- a/third_party/WebKit/Source/core/layout/ng/ng_layout_opportunity_iterator.cc
+++ b/third_party/WebKit/Source/core/layout/ng/ng_layout_opportunity_iterator.cc
@@ -148,6 +148,10 @@ NGConstraintSpace* GetTopSpace(const NGConstraintSpace& space,
 void InsertExclusion(NGLayoutOpportunityTreeNode* node,
                      const NGExclusion* exclusion,
                      NGLayoutOpportunities& opportunities) {
+  // Base case: there is no node.
+  if (!node)
+    return;
+
   // Base case: exclusion is not in the node's constraint space.
   if (!IsExclusionWithinSpace(*node->space, *exclusion))
     return;
@@ -216,9 +220,9 @@ NGLayoutOpportunityIterator::NGLayoutOpportunityIterator(
 
   opportunity_tree_root_ = new NGLayoutOpportunityTreeNode(space);
   for (const auto exclusion : exclusions) {
-    InsertExclusion(opportunity_tree_root_, exclusion, opportunities_);
+    InsertExclusion(MutableOpportunityTreeRoot(), exclusion, opportunities_);
   }
-  CollectAllOpportunities(opportunity_tree_root_, opportunities_);
+  CollectAllOpportunities(OpportunityTreeRoot(), opportunities_);
   std::sort(opportunities_.begin(), opportunities_.end(),
             &CompareNGLayoutOpportunitesByStartPoint);
   opportunity_iter_ = opportunities_.begin();