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The CL adds MetadataCacheSet class that is the set of MetadataCacheItem.  The
class wraps MetadataCacheSetStorage interface (raw set of MetadataCacheItem),
and prvoides operations to the set of MetadataCacheItem.

BUG=410766
TEST=FileManagerJsTest.MetadataCacheSetTest

Review URL: https://codereview.chromium.org/894633002

Cr-Commit-Position: refs/heads/master@{#314125}