[new downloader] Adding storage_helpers.

storage/storage_helpers.cpp

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+#include "storage/storage_helpers.hpp"
+
+#include "storage/country_info_getter.hpp"
+#include "storage/storage.hpp"
+
+namespace storage
+{
+
+bool IsPointCoveredByDownloadedMaps(m2::PointD const & position,
+                                    Storage const & storage,
+                                    CountryInfoGetter const & countryInfoGetter)
+{
+  return storage.IsNodeDownloaded(countryInfoGetter.GetRegionCountryId(position));
+}
+
+} // namespace storage

storage/storage_helpers.hpp

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+#pragma once
+
+#include "geometry/point2d.hpp"
+
+namespace storage
+{
+class CountryInfoGetter;
+class Storage;
+
+/// \returns true if |position| is covered by a downloaded mwms and false otherwise.
+/// \note |position| has coordinates in mercator.
+/// \note This method takes into acount only maps enumerated in countries.txt.
+bool IsPointCoveredByDownloadedMaps(m2::PointD const & position,
+                                    Storage const & storage,
+                                    CountryInfoGetter const & countryInfoGetter);
+
+} // namespace storage