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This CL is the fist in a series to convert DataItem from the deprecated
base::Callback (and Bind) to [Once|Repeating]Callback. See the relation
chain of CLs for the complete change.

This CL starts by converting a set of injectable callbacks used for unit
tests to observe this class. These callbacks may be called repeatedly
after being registered so they are RepeatingCallbacks. However,
ItemFactoryCallback and ItemStoreDeleter both take a callback as a
parameter. The unit test implementations only invoke these parameters
once so we convert them to the "Once" variant.

The RegisteredValuesCallback is a type defined in DataItem and used
elsewhere. For now, we add a "Once" typedef to prevent changing too
much. This will be removed in a downstream patch once all uses of the
ambiguous form are removed.

Bug: 1152268
Change-Id: I694df544b97ede7b1e5aa8af388ca4ed186d8156
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2595932Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838226}