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The class allows legacy datatypes (non-USS), which implement
SyncableService, to be integrated within the USS architecture, making
it possible to reuse central objects like
ClientTagBasedModelTypeProcessor and ModelTypeWorker.

Design Doc (Googlers only):
https://docs.google.com/document/d/14ScYZ0sop921gjBwXuReIEuQJlwftqkuSM1jMK_A_x4

In this first patch, we introduce the bridge itself. Follow-up patches
will introduce the necessary plumbing to exercise it.

Expected main benefits:
- Unify logic across datatypes by avoiding subtle behavioral differences
  across architectures.
- Remove lots of code (~25K LoC), hence:
  a) reduce Chrome binary size.
  b) reduce maintenance cost.
  c) eng ramp-up time.
- Improve resource footprint (less RAM, ~50% savings).
- Reduce the gap for datatypes to actually migrate to USS.
- Unblock multiple cleanup work, including a massive simplification of
  DataTypeManager and related classes, including controllers.

Bug: 870624
Change-Id: I1bd7f553bf22886c5136e7e12f13b37a3dc77a39
Reviewed-on: https://chromium-review.googlesource.com/1164742Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595017}