Rename SyncBackendHostCore->SyncEngineBackend

Trivially rename the class in accordance to more modern naming
conventions, in particular SyncEngineImpl.

Bug: 950852
Change-Id: Iea1761ef3866bcf25e1373e6bb3a83a2fac29fdf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1559312
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649031}
10 files changed