Blimp: Add BlobChannel Helium messages and delegate impls.

blimp/common/blob_cache/blob_cache.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 BLIMP_COMMON_BLOB_CACHE_BLOB_CACHE_H_
 #define BLIMP_COMMON_BLOB_CACHE_BLOB_CACHE_H_
 
 #include <string>
 #include <vector>
 
 #include "base/memory/ref_counted.h"
 #include "blimp/common/blimp_common_export.h"
 
 namespace blimp {
 
 using BlobId = std::string;
-using BlobData = base::RefCountedData<const std::string>;
-using BlobDataPtr = scoped_refptr<BlobData>;
+using BlobData = base::RefCountedData<std::string>;
+
+// Immutable, ref-counted representation of blob payloads, suitable for sharing
+// across threads.
+using BlobDataPtr = scoped_refptr<const BlobData>;
 
 // An interface for a cache of blobs.
 class BLIMP_COMMON_EXPORT BlobCache {
  public:
   virtual ~BlobCache() {}
 
   // Returns true if there is a cache item |id|.
   virtual bool Contains(const BlobId& id) const = 0;
 
   // Stores |data| with the identifier |id| in the cache.
   // Command is ignored if there is already a cache item stored under |id|.
   virtual void Put(const BlobId& id, BlobDataPtr data) = 0;
 
   // Returns a pointer to the cache item |id|, or nullptr if no cache item
   // exists under that identifier.
   virtual BlobDataPtr Get(const BlobId& id) const = 0;
 };
 
 }  // namespace blimp
 
 #endif  // BLIMP_COMMON_BLOB_CACHE_BLOB_CACHE_H_