Add metrics for the Clear-Site-Data header.

content/browser/browsing_data/clear_site_data_throttle.h

Index: content/browser/browsing_data/clear_site_data_throttle.h
diff --git a/content/browser/browsing_data/clear_site_data_throttle.h b/content/browser/browsing_data/clear_site_data_throttle.h
index ea5996679cec3803605e9f0a5565586482f6e7d9..024982754cbb274f2a2e1fca284afd1494bea9e1 100644
--- a/content/browser/browsing_data/clear_site_data_throttle.h
+++ b/content/browser/browsing_data/clear_site_data_throttle.h
@@ -76,6 +76,10 @@ class CONTENT_EXPORT ClearSiteDataThrottle : public NavigationThrottle {
   // been completed;
   bool clearing_in_progress_;
 
+  // The time when the last clearing operation started. Used when clearing
+  // finishes to compute the duration.
+  base::TimeTicks clearing_started_;
+
   // Needed for asynchronous parsing and deletion tasks.
   base::WeakPtrFactory<ClearSiteDataThrottle> weak_ptr_factory_;