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This CL is a refactoring that should contain no behavioural changes.
It splits parts of the DigitalGoodsConverter into two additional
classes, the GetDetailsConverter and the AcknowledgeConverter, and it
deduplicates some of the logic in the DigitalGoodsAdapter.

This should put the code in a slightly nicer state to add the new
method call, ListPurchases.

Change-Id: I6e56a5dc2077371a8de8bbb16eb0799bdb971a31
Bug: 1139795
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487087Reviewed-by: Ella Ge <eirage@chromium.org>
Commit-Queue: Peter Conn <peconn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819898}