Adding DRP specfic UMA for FirstContentfulPaint

chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.h

Index: chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.h
diff --git a/chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.h b/chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.h
index 40224e6e7558a0b04ded89e91db6baf49223bd42..5bf5f650e03fc4e53a4719fc632acf3a2c1af97c 100644
--- a/chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.h
+++ b/chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.h
@@ -35,20 +35,23 @@ extern const char kHistogramFirstContentfulPaintLow[];
 
 extern const char kHistogramFirstBackground[];
 extern const char kHistogramFirstForeground[];
 
 extern const char kHistogramBackgroundBeforePaint[];
 extern const char kHistogramBackgroundBeforeCommit[];
 extern const char kHistogramFailedProvisionalLoad[];
 
 extern const char kRapporMetricsNameCoarseTiming[];
 
+extern const char kHistogramFirstContentfulPaintDataReductionProxy[];
+extern const char kHistogramFirstContentfulPaintDataReductionProxyAutoLofiOn[];
+extern const char kHistogramFirstContentfulPaintDataReductionProxyAutoLofiOff[];
 }  // namespace internal
 
 // Observer responsible for recording 'core' page load metrics. Core metrics are
 // maintained by loading-dev team, typically the metrics under
 // PageLoad.Timing2.*.
 class CorePageLoadMetricsObserver
     : public page_load_metrics::PageLoadMetricsObserver {
  public:
   CorePageLoadMetricsObserver();
   ~CorePageLoadMetricsObserver() override;