Change NGLayoutOpportunityIterator constructor to work with const CS.

third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.h

Index: third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.h
diff --git a/third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.h b/third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.h
index 215cbc68c4d96174e66df45cd33701032d5290ef..1491b544c9ddff906ff2c23fc7651d220eea91a8 100644
--- a/third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.h
+++ b/third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.h
@@ -77,6 +77,9 @@ class CORE_EXPORT NGBlockLayoutAlgorithm : public NGLayoutAlgorithm {
 
   const ComputedStyle& Style() const { return node_->Style(); }
 
+  // Mutable Getters.
+  NGConstraintSpace* MutableConstraintSpace() { return constraint_space_; }
+
   Persistent<NGBlockNode> node_;
   NGConstraintSpace* constraint_space_;