[BlobReader] Correct IsInMemory = false when async constructing

storage/browser/blob/blob_reader.h

 // Copyright 2015 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 STORAGE_BROWSER_BLOB_BLOB_READER_H_
 #define STORAGE_BROWSER_BLOB_BLOB_READER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 102 matching lines @@
   //   Status::DONE) of 0 means we're finished reading.
   Status Read(net::IOBuffer* buffer,
               size_t dest_size,
               int* bytes_read,
               net::CompletionCallback done);
 
   // Kills reading and invalidates all callbacks. The reader cannot be used
   // after this call.
   void Kill();
 
-  // Returns if all of the blob's items are in memory.
+  // Returns if all of the blob's items are in memory. Should only be called
+  // after CalculateSize.
   bool IsInMemory() const;
 
   // Returns the remaining bytes to be read in the blob. This is populated
   // after CalculateSize, and is modified by SetReadRange.
   uint64_t remaining_bytes() const { return remaining_bytes_; }
 
   // Returns the net error code if there was an error. Defaults to net::OK.
   int net_error() const { return net_error_; }
 
   // Returns the total size of the blob. This is populated after CalculateSize
@@ skipping 87 matching lines @@
 
   net::CompletionCallback size_callback_;
   net::CompletionCallback read_callback_;
 
   base::WeakPtrFactory<BlobReader> weak_factory_;
   DISALLOW_COPY_AND_ASSIGN(BlobReader);
 };
 
 }  // namespace storage
 #endif  // STORAGE_BROWSER_BLOB_BLOB_READER_H_