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The class will be used as the base class of all metadata provider.  The class has abstrct
getImpl method and sub-class of the class should inherit the function. The class
also contains MetadataProviderCache that is a sub-class of MetadataCacheSet, and
caches the result of getImpl method.

BUG=410766
TEST=FileManagerJsTest.NewMetadataProvider

Review URL: https://codereview.chromium.org/894283002

Cr-Commit-Position: refs/heads/master@{#314279}