Change NGLayoutOpportunityIterator constructor to work with const CS.

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NGBlockLayoutAlgorithm_h
 #define NGBlockLayoutAlgorithm_h
 
 #include "core/CoreExport.h"
 #include "core/layout/ng/ng_block_node.h"
 #include "core/layout/ng/ng_break_token.h"
@@ skipping 59 matching lines @@
     return NGLogicalOffset(border_and_padding_.inline_start, content_size_);
   }
 
   const NGConstraintSpace& ConstraintSpace() const {
     DCHECK(constraint_space_);
     return *constraint_space_;
   }
 
   const ComputedStyle& Style() const { return node_->Style(); }
 
+  // Mutable Getters.
+  NGConstraintSpace* MutableConstraintSpace() { return constraint_space_; }
+
   Persistent<NGBlockNode> node_;
   NGConstraintSpace* constraint_space_;
 
   // The break token from which we are currently resuming layout.
   Persistent<NGBlockBreakToken> break_token_;
 
   NGFragmentBuilder builder_;
   NGConstraintSpaceBuilder space_builder_;
 
   NGBoxStrut border_and_padding_;
   LayoutUnit content_size_;
   LayoutUnit max_inline_size_;
   // MarginStrut for the previous child.
   NGMarginStrut curr_margin_strut_;
   NGLogicalOffset curr_bfc_offset_;
   NGBoxStrut curr_child_margins_;
 };
 
 }  // namespace blink
 
 #endif  // NGBlockLayoutAlgorithm_h