Adding previews information to PLM UKM

chrome/browser/page_load_metrics/page_load_metrics_observer.h

 // Copyright 2015 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 CHROME_BROWSER_PAGE_LOAD_METRICS_PAGE_LOAD_METRICS_OBSERVER_H_
 #define CHROME_BROWSER_PAGE_LOAD_METRICS_PAGE_LOAD_METRICS_OBSERVER_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 433 matching lines @@
   // commit, immediately before the observer is deleted.
   virtual void OnFailedProvisionalLoad(
       const FailedProvisionalLoadInfo& failed_provisional_load_info,
       const PageLoadExtraInfo& extra_info) {}
 
   // Called whenever a request is loaded for this page load. This comes
   // unfiltered from the ResourceDispatcherHost and may include blob requests
   // and data uris.
   virtual void OnLoadedResource(
       const ExtraRequestCompleteInfo& extra_request_complete_info) {}
+
+  // Called when the event corresponding to |event_key| occurs in this page
+  // load.
+  virtual void OnEventOccurred(const void* const event_key) {}
 };
 
 }  // namespace page_load_metrics
 
 #endif  // CHROME_BROWSER_PAGE_LOAD_METRICS_PAGE_LOAD_METRICS_OBSERVER_H_