Add PageLoad metrics for multi-tab loading cases

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 93ad46aa58d71e09c8a6b2c5f14073f239a2047b..c6d9e736ad0646f8db895bdc9c0113aaef908d8f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -92285,6 +92285,10 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
       name="PageLoad.Clients.DocWrite.Evaluator.Timing2.ParseBlockedOnScriptLoadFromDocumentWrite"/>
   <affected-histogram
       name="PageLoad.Clients.DocWrite.Evaluator.Timing2.ParseDuration"/>
+  <affected-histogram
+      name="PageLoad.Clients.MultiTabLoading.DocumentTiming.NavigationToDOMContentLoadedEventFired"/>
+  <affected-histogram
+      name="PageLoad.Clients.MultiTabLoading.DocumentTiming.NavigationToLoadEventFired"/>
   <affected-histogram
       name="PageLoad.Clients.ServiceWorker.PaintTiming.NavigationToFirstContentfulPaint"/>
   <affected-histogram
@@ -92669,6 +92673,25 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <affected-histogram name="PageLoad.Experimental.Bytes.Total"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="PageLoadMetricsClientsMultiTabLoading" separator="."
+    ordering="prefix">
+  <suffix name="Clients.MultiTabLoading"
+      label="PageLoadMetrics for page loads that started while there are
+             other loading tabs."/>
+  <affected-histogram
+      name="PageLoad.DocumentTiming.NavigationToDOMContentLoadedEventFired"/>
+  <affected-histogram
+      name="PageLoad.DocumentTiming.NavigationToLoadEventFired"/>
+  <affected-histogram
+      name="PageLoad.Experimental.PaintTiming.ForegroundToFirstMeaningfulPaint"/>
+  <affected-histogram
+      name="PageLoad.Experimental.PaintTiming.NavigationToFirstMeaningfulPaint"/>
+  <affected-histogram
+      name="PageLoad.PaintTiming.ForegroundToFirstContentfulPaint"/>
+  <affected-histogram
+      name="PageLoad.PaintTiming.NavigationToFirstContentfulPaint"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="PageLoadMetricsClientsNoServiceWorkerSpecialApps"
     separator=".">
   <suffix name="search"