Add UKM metric tracking for various page load metric observers.

services/metrics/public/cpp/ukm_recorder.h

Index: services/metrics/public/cpp/ukm_recorder.h
diff --git a/services/metrics/public/cpp/ukm_recorder.h b/services/metrics/public/cpp/ukm_recorder.h
index 1aadef76bfdfa3d1d5485a4ecb39add39c885f8c..5e6e8b1ebf60189d098ceff7450f474ac1741549 100644
--- a/services/metrics/public/cpp/ukm_recorder.h
+++ b/services/metrics/public/cpp/ukm_recorder.h
@@ -19,7 +19,11 @@
 #include "url/gurl.h"
 
 class ContextualSearchRankerLoggerImpl;
+class DocumentWritePageLoadMetricsObserver;
+class FromGWSPageLoadMetricsLogger;
 class PluginInfoMessageFilter;
+class ServiceWorkerPageLoadMetricsObserver;
+class SubresourceFilterMetricsObserver;
 class UkmPageLoadMetricsObserver;
 
 namespace autofill {
@@ -90,6 +94,10 @@ class METRICS_EXPORT UkmRecorder {
   friend ContextualSearchRankerLoggerImpl;
   friend PluginInfoMessageFilter;
   friend UkmPageLoadMetricsObserver;
+  friend DocumentWritePageLoadMetricsObserver;
+  friend FromGWSPageLoadMetricsLogger;
+  friend ServiceWorkerPageLoadMetricsObserver;
+  friend SubresourceFilterMetricsObserver;
   friend translate::TranslateRankerImpl;
   friend TestRecordingHelper;
   friend UkmInterface;