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All datatypes (except proxy types) are non-blocking, so there is little
point in maintaining code that keeps track of which precise "group" each
datatype maps to.

In this patch, most code is deleted except for the internals of
SyncBackendRegistrar, which is more complicated.

Change-Id: I4ad2d219e8032cbb0dca16125958c6e66c117cf6
Bug: 1102835
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2467918
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816676}