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According to profiling the first two read passes over SyncData DB
are from DirectoryBackingStore::DropDeletedEntries. This function
executes two SQL queries that drop entries pending deletion.
Since it is uncommon to have entries pending deletion it is far
more efficient to remove DirectoryBackingStore::DropDeletedEntries
and instead check for the entries matching this criteria inside
DirectoryBackingStore::LoadEntries, skip those entries from being
loaded and put them straight into Directory's metahandles_to_purge
collection which sets them up for deletion during a subsequent database
save.

See the bug for the preliminary performance results.

BUG=464073

Review URL: https://codereview.chromium.org/1008103002

Cr-Commit-Position: refs/heads/master@{#320861}