SQLite WAL mode by default checkpoints the WAL to the main database at
close time.  Chromium closes databases at shutdown, which causes
shutdown slowdowns due to buffered data being flushed.  WAL
checkpointing would effectively increase the amount of buffered data
being flushed.

[WAL mode is currently not in use by Chromium.]

BUG=675264

Review-Url: https://codereview.chromium.org/2827673006
Cr-Commit-Position: refs/heads/master@{#465772}