Add UMA to measure how often pages don't receive any user input

chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc

Index: chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc
diff --git a/chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc b/chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc
index 98b46899a0d8fe5feb3ffa75c4fad1f293efff0e..f7cd13ed24b471303500de38bfb70b512d184284 100644
--- a/chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc
+++ b/chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc
@@ -189,6 +189,8 @@ const char kHistogramFirstContentfulPaintUserInitiated[] =
 
 const char kHistogramFirstMeaningfulPaintStatus[] =
     "PageLoad.Experimental.PaintTiming.FirstMeaningfulPaintStatus";
+const char kHistogramHadUserInputAfterFirstPaint[] =
+    "PageLoad.Experimental.PaintTiming.HadUserInputAfterFirstPaint";
 
 const char kHistogramFirstNonScrollInputAfterFirstPaint[] =
     "PageLoad.InputTiming.NavigationToFirstNonScroll.AfterPaint";
@@ -565,6 +567,8 @@ void CorePageLoadMetricsObserver::RecordTimingHistograms(
         internal::FIRST_MEANINGFUL_PAINT_DID_NOT_REACH_NETWORK_STABLE :
         internal::FIRST_MEANINGFUL_PAINT_DID_NOT_REACH_FIRST_CONTENTFUL_PAINT);
   }
+  UMA_HISTOGRAM_BOOLEAN(internal::kHistogramHadUserInputAfterFirstPaint,
+                        !first_user_interaction_after_first_paint_.is_null());
 }
 
 void CorePageLoadMetricsObserver::RecordRappor(