Add FMP to DataReductionProxy UMA and to Pingback Metrics

components/data_reduction_proxy/proto/pageload_metrics.proto

Index: components/data_reduction_proxy/proto/pageload_metrics.proto
diff --git a/components/data_reduction_proxy/proto/pageload_metrics.proto b/components/data_reduction_proxy/proto/pageload_metrics.proto
index 641789f01dcbbc346421833fba6384268d09e8c8..cd9d1420b257f14a4008ccbede0afe2484091f05 100644
--- a/components/data_reduction_proxy/proto/pageload_metrics.proto
+++ b/components/data_reduction_proxy/proto/pageload_metrics.proto
@@ -61,11 +61,15 @@ message PageloadMetrics {
   optional int32 compressed_page_size_bytes = 11;
 
   // The effective connection type at the start of the navigation.
   optional EffectiveConnectionType effective_connection_type = 12;
 
   // The amount of time the parser was blocked by loading script.
   optional Duration parse_blocked_on_script_load_duration = 13;
 
   // Time until parsing finished.
   optional Duration parse_stop = 14;
+
+  // Time to first meaningful paint. This measure is unstable and will change
+  // over time.
+  optional Duration experimental_time_to_first_meaningful_paint = 15;
 }