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New MetadataCache enables us to load metadata property one by one.  The CL adds
MetadataCacheItem (for each FileEntry), MetadataCacheItemProperty (for each
property) to manage cache state of each property.

BUG=410766
TEST=FileManagerJsTest.MetadataCacheItemTest

Review URL: https://codereview.chromium.org/869923004

Cr-Commit-Position: refs/heads/master@{#314116}