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This change introduces SyncServiceBase as a base class of PSS and
consolidates engine initialization logic into it from PSS and SBHI.

This new class will enable us to siphon logic out of SBHI until it can
become SyncEngineProxy (no business logic). This change also begins
aligning SBHC with the SyncEngine interface, so it can eventually become
SyncEngineImpl.

BUG=669967

Review-Url: https://codereview.chromium.org/2559123002
Cr-Commit-Position: refs/heads/master@{#438620}