Modify SpdyHeaderBlock's internals to consolidate header values only on first access.

net/base/arena.h

 // Copyright (c) 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 NET_BASE_ARENA_H_
 #define NET_BASE_ARENA_H_
 
 #include <memory>
 #include <vector>
 
 #include "net/base/net_export.h"
 
 namespace net {
 
 // Allocates large blocks of memory, and doles them out in smaller chunks.
 // Not thread-safe.
 class NET_EXPORT_PRIVATE UnsafeArena {
  public:
+  class Status {
+   private:
+    friend class UnsafeArena;
+    size_t bytes_allocated_;
+
+   public:
+    Status() : bytes_allocated_(0) {}
+    size_t bytes_allocated() const { return bytes_allocated_; }
+  };
+
   // Blocks allocated by this arena will be at least |block_size| bytes.
   explicit UnsafeArena(size_t block_size);
   ~UnsafeArena();
 
   // Copy and assign are not allowed.
   UnsafeArena() = delete;
   UnsafeArena(const UnsafeArena&) = delete;
   UnsafeArena& operator=(const UnsafeArena&) = delete;
 
   // Move is allowed.
   UnsafeArena(UnsafeArena&& other);
   UnsafeArena& operator=(UnsafeArena&& other);
 
   char* Alloc(size_t size);
   char* Realloc(char* original, size_t oldsize, size_t newsize);
   char* Memdup(const char* data, size_t size);
 
   // If |data| and |size| describe the most recent allocation made from this
   // arena, the memory is reclaimed. Otherwise, this method is a no-op.
   void Free(char* data, size_t size);
 
   void Reset();
 
+  Status status() const { return status_; }
+
  private:
   struct Block {
     std::unique_ptr<char[]> data;
     size_t size = 0;
     size_t used = 0;
 
     explicit Block(size_t s);
     ~Block();
 
     Block(Block&& other);
     Block& operator=(Block&& other);
   };
 
   void Reserve(size_t additional_space);
   void AllocBlock(size_t size);
 
   size_t block_size_;
   std::vector<Block> blocks_;
+  Status status_;
 };
 
 }  // namespace net
 
 #endif  // NET_BASE_ARENA_H_