Implement File constructor.

Source/core/fileapi/Blob.h

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 42 matching lines @@
     static PassRefPtr<Blob> create(PassRefPtr<BlobDataHandle> blobDataHandle)
     {
         return adoptRef(new Blob(blobDataHandle));
     }
 
     virtual ~Blob();
 
     String uuid() const { return m_blobDataHandle->uuid(); }
     String type() const {  return m_blobDataHandle->type(); }
     virtual unsigned long long size() const { return m_blobDataHandle->size(); }
+    // True for all File instances, including the user-built ones.
     virtual bool isFile() const { return false; }
+    // Only true for File instances that are backed by platform files.
+    virtual bool hasBackingFile() const { return false; }
     PassRefPtr<BlobDataHandle> blobDataHandle() const { return m_blobDataHandle; }
     PassRefPtr<Blob> slice(long long start = 0, long long end = std::numeric_limits<long long>::max(), const String& contentType = String()) const;
 
     // URLRegistrable to support PublicURLs.
     virtual URLRegistry& registry() const OVERRIDE;
 
 protected:
     explicit Blob(PassRefPtr<BlobDataHandle>);
 
 private:
     Blob();
     RefPtr<BlobDataHandle> m_blobDataHandle;
 };
 
 } // namespace WebCore
 
 #endif // Blob_h