[Payments] Use event to log checkout funnel metrics.

components/payments/core/journey_logger.h

Index: components/payments/core/journey_logger.h
diff --git a/components/payments/core/journey_logger.h b/components/payments/core/journey_logger.h
index a7e72bd81f39bab1e71035df8571c32aa65bdfb0..5510cf9a74afe9b8768511d7168c016503cf637a 100644
--- a/components/payments/core/journey_logger.h
+++ b/components/payments/core/journey_logger.h
@@ -136,6 +136,10 @@ class JourneyLogger {
     bool is_requested_;
   };
 
+  // Records the histograms for all the steps of a successful checkout flow

[gogerald1, 2017/05/19 19:42:17]

nit: *a complete*

+  // that were reached.
+  void RecordCheckoutFlowMetrics();
+
   // Records the histograms for all the sections that were requested by the
   // merchant.
   void RecordSectionSpecificStats(CompletionStatus completion_status);