Record start up latency of DOM distiller viewer

components/dom_distiller/content/browser/dom_distiller_viewer_source.cc

Index: components/dom_distiller/content/browser/dom_distiller_viewer_source.cc
diff --git a/components/dom_distiller/content/browser/dom_distiller_viewer_source.cc b/components/dom_distiller/content/browser/dom_distiller_viewer_source.cc
index 31735df89753dd807c896debdf04c61085b4288b..12f679d322d34b091773a4bd7a22e9229c0e9253 100644
--- a/components/dom_distiller/content/browser/dom_distiller_viewer_source.cc
+++ b/components/dom_distiller/content/browser/dom_distiller_viewer_source.cc
@@ -11,7 +11,7 @@
 
 #include "base/memory/ref_counted_memory.h"
 #include "base/message_loop/message_loop.h"
-#include "base/metrics/user_metrics.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
@@ -157,6 +157,15 @@ void DomDistillerViewerSource::RequestViewerHandle::DidFinishLoad(
     return;
   }
 
+  int64_t start_time_ms = url_utils::GetTimeFromDistillerUrl(validated_url);
+  if (start_time_ms > 0) {
+    base::TimeTicks start_time =
+        base::TimeDelta::FromMilliseconds(start_time_ms) + base::TimeTicks();
+    base::TimeDelta latency = base::TimeTicks::Now() - start_time;
+
+    UMA_HISTOGRAM_TIMES("DomDistiller.Time.ViewerLoading", latency);
+  }
+
   // No SendJavaScript() calls allowed before |buffer_| is run and cleared.
   waiting_for_page_ready_ = false;
   if (!buffer_.empty()) {