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GetDataTypesFromPreferenceProviders was identical to GetForcedDataTypes,
so let's merge it into that.
This also includes a tiny behavior change: GetForcedDataTypes will now
return only types which are registered. Unregistered types effectively
don't exist, so no point in considering them forced.

Bug: none
Change-Id: I37d5d105ea88f80b25f7b651368a720d3f915fba
Reviewed-on: https://chromium-review.googlesource.com/c/1314490Reviewed-by: vitaliii <vitaliii@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604901}