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This pipes the call to GetProperty from ProtoDatabase through to
leveldb_proto LevelDB and finally onto the underlying Level DB, so we
can retrieve the value for tests and metrics.

Bug: 870813
Change-Id: I23f89685017eeae77e19c37847d6399cc8d6085f
Reviewed-on: https://chromium-review.googlesource.com/1169878
Commit-Queue: Troy Hildebrandt <thildebr@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585252}