Reporting: Document browsing data remover + endpoint backoff

The ReportingBrowsingDataRemover currently doesn't remove entries from
the endpoint backoff map in the ReportingEndpointManager, because it's
never persisted to disk. This CL adds comments in a couple places to
note this, so someone implementing persistence for that knows to
implement browsing data removal as well.

BUG=704259,676324

Review-Url: https://codereview.chromium.org/2862003003
Cr-Commit-Position: refs/heads/master@{#470336}

net/reporting/reporting_browsing_data_remover.h

@@ -35,6 +35,10 @@ class NET_EXPORT ReportingBrowsingDataRemover {
   // which types of data to remove: reports queued by browser features and/or
   // clients (endpoints configured by origins). |origin_filter|, if not null,
   // specifies which origins' data to remove.
+  //
+  // Note: Currently this does not clear the endpoint backoff data in
+  // ReportingEndpointManager because that's not persisted to disk. If it's ever
+  // persisted, it will need to be cleared as well.
   virtual void RemoveBrowsingData(
       int data_type_mask,
       base::Callback<bool(const GURL&)> origin_filter) = 0;

net/reporting/reporting_endpoint_manager.h

@@ -72,6 +72,10 @@ class NET_EXPORT ReportingEndpointManager {
   ReportingContext* context_;
 
   std::set<GURL> pending_endpoints_;
+
+  // Note: Currently the ReportingBrowsingDataRemover does not clear this data
+  // because it's not persisted to disk. If it's ever persisted, it will need
+  // to be cleared as well.
   std::map<GURL, std::unique_ptr<net::BackoffEntry>> endpoint_backoff_;
 
   DISALLOW_COPY_AND_ASSIGN(ReportingEndpointManager);