UMA histogram for longest renderer main thread usage for page save to MHTML.

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 636288cc0daa19060127118558e4e44022b0a6bf..fc6ee09ef934541dbc927e4d3296c99aff5ac512 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -40764,6 +40764,19 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram
+    name="PageSerialization.MhtmlGeneration.RendererMainThreadTime.SlowestFrame"
+    units="ms">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    The longest time taken by the main thread of a render processes to save one
+    frame of a page being saved to MHTML. In other words this is the maximum
+    value reported in
+    PageSerialization.MhtmlGeneration.RendererMainThreadTime.SingleFrame for one
+    page save operation.
+  </summary>
+</histogram>
+
+<histogram
     name="PageSerialization.MhtmlGeneration.SerializationTime.SingleFrame"
     units="microseconds">
   <owner>carlosk@chromium.org</owner>