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Currently, we use DictionaryValues in the Physical Web data source.
The problem with this is that we cannot store store longs (timestamps).
This change introduces a metadata struct that can be requested from the
data source, deprecating the DictionaryValue option.

BUG=667722

Review-Url: https://codereview.chromium.org/2561493002
Cr-Commit-Position: refs/heads/master@{#443425}