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Keys and entries can currently be loaded separately, but never at the
same time, meaning it requires separate calls to LoadKeys/LoadEntries
to get keys and entries. Some users of ProtoDatabase have resorted to
storing the keys in the entries to get around this.

This CL adds LoadKeysAndEntries(WithFilter) that returns both at the
same time. ProtoDatabase's new LoadKeysAndEntriesCallback gives back a
vector of string/proto pairs.

Bug: 883409
Change-Id: Id624bbdeb356395c6ce679528d85c1f934a9772e
Reviewed-on: https://chromium-review.googlesource.com/1228305
Commit-Queue: Troy Hildebrandt <thildebr@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595175}