Add explicit destructor to DatabaseBackend.h.

Source/modules/webdatabase/DatabaseBackend.h

Index: Source/modules/webdatabase/DatabaseBackend.h
diff --git a/Source/modules/webdatabase/DatabaseBackend.h b/Source/modules/webdatabase/DatabaseBackend.h
index a543ddc0e6992e2fef32322a20acfdb10b5203a9..8a71d361a07ac79118a0c56e84f1822f9340ad43 100644
--- a/Source/modules/webdatabase/DatabaseBackend.h
+++ b/Source/modules/webdatabase/DatabaseBackend.h
@@ -49,6 +49,7 @@ class SQLTransactionCoordinator;
 class DatabaseBackend : public DatabaseBackendBase {
 public:
     DatabaseBackend(DatabaseContext*, const String& name, const String& expectedVersion, const String& displayName, unsigned long estimatedSize);
+    virtual ~DatabaseBackend();
     virtual void trace(Visitor*) OVERRIDE;
 
     virtual bool openAndVerifyVersion(bool setVersionInNewDatabase, DatabaseError&, String& errorMessage) OVERRIDE FINAL;