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IDBCursor.Advance() previously took a separate IDBCallbacks interface
that had its own lifecycle and could have a number of methods called
on it.

This change updates Advance() to use Mojo's native callback mechanism
so we can start moving away from the complicated IDBCallbacks
interface and instead use a simpler and easier to reason about async
response return approach.

Bug: 717812
Change-Id: Ia08bebfcafe5f63d60366e81b7f973b1e9ad2378
Reviewed-on: https://chromium-review.googlesource.com/c/1336662
Commit-Queue: Chase Phillips <cmp@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622176}