Create Field Trial param for whitelisting UKM Entries.

components/ukm/ukm_entry.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_UKM_UKM_ENTRY_H_
 #define COMPONENTS_UKM_UKM_ENTRY_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 11 matching lines @@
 // proto message, the event and metric names will be hashed by
 // base::HashMetricName.
 //
 // To build UkmEntry objects, please use UkmEntryBuilder.
 class UkmEntry {
  public:
   // Serializes the members of the class into the supplied proto.
   void PopulateProto(Entry* proto_entry) const;
 
   int32_t source_id() const { return source_id_; }
+  uint64_t event_hash() const { return event_hash_; }
 
   ~UkmEntry();
 
  private:
   friend UkmEntryBuilder;
 
   UkmEntry(int32_t source_id, const char* event_name);
 
   const int32_t source_id_;
   const uint64_t event_hash_;
   std::vector<std::pair<uint64_t, int64_t>> metrics_;
 
   DISALLOW_COPY_AND_ASSIGN(UkmEntry);
 };
 
 }  // namespace ukm
 
 #endif  // COMPONENTS_UKM_UKM_ENTRY_H_