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This CL adds a PersistentReportingStore* to the constructor of
ReportingService. This pointer is plumbed down to the ReportingCache,
which will eventually use the store to persist its data to disk.
The store must outlive the ReportingService. If null, the
ReportingCache will be in-memory only. The ReportingService is
instantiated with a nullptr placeholder as the store for now.

Bug: 895821
Change-Id: I3a3234653707d2dd87f475413c6318ee7b43121a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1575079
Commit-Queue: Lily Chen <chlily@chromium.org>
Reviewed-by: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#656973}