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Previously, when saving items, the new metadata was generated by
GalleryDataModel. But the generated metadata was broken and it contained old
thumbnail data that causes memory shortage.

The metadata for new items can be otained by using normal code path
(MetadataCache), so the CL removes special geneartion of metadata for new items
and let Gallery obtain the metadata by using MeatadataCache.

BUG=439898
TEST=GalleryJsTest.GalleryItemTest

Review URL: https://codereview.chromium.org/802283005

Cr-Commit-Position: refs/heads/master@{#310205}