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The corresponding data types have been migrated to USS, which uses
GROUP_NON_BLOCKING (which essentially means "don't use a
ModelSafeWorker at all"). So the workers for FILE and HISTORY were
still instantiated, but never actually used.
This also lets us remove the whole HistoryModelWorker class.

TBR=sky
for trivial updates to components/history/core/browser/BUILD.gn
(removing the deleted sync files).

Bug: 923287
Change-Id: Ief4ed46fce41346ea35ea46535b8d06d6c4d212e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1720709
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681746}