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The collection store will be used by notification store and impression
store. They both loads data into memory and keep in-memory data synced
with the underlying storage.

The icon store will not use this because it only moves data through the
pipeline and never held by classes in memory.

This CL tweaks the collection store interface to better fit the need
of notification store and impression store.

Bug: 930968
Change-Id: Ia113c3ca9a4074f1673c957e08e82fda6208bdd5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1548633Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646636}