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By using templates the database is reusable for different protocol
buffers. One application is for chrome cart which in the experimental
code looked very similar to persisted_state_db. By using templates we
can avoid duplicating a lot of code.

Bug: 1154499
Change-Id: I65bdaa3142eede152db26cd5629001b815f3b493
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2570451Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: ssid <ssid@chromium.org>
Reviewed-by: Yue Zhang <yuezhanggg@chromium.org>
Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: David Maunder <davidjm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836924}