Fail when saving page as MHTML provides information about the cause.

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 2b27cb685bbb799ba897cf6f4ca8a0e6071ab96e..4071ce87e468e59444c9df94909d3ece91db546e 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -42142,6 +42142,12 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PageSerialization.MhtmlGeneration.FinalSaveStatus"
+    enum="MhtmlGenerationFinalSaveStatus">
+  <owner>carlosk@chromium.org</owner>
+  <summary>Final status of the MHTML save operation for a page.</summary>
+</histogram>
+
 <histogram name="PageSerialization.MhtmlGeneration.FullPageSavingTime"
     units="ms">
   <owner>carlosk@chromium.org</owner>
@@ -93366,6 +93372,16 @@ value.
   <int value="2" label="Enabled successfully"/>
 </enum>
 
+<enum name="MhtmlGenerationFinalSaveStatus" type="int">
+  <int value="0" label="Success"/>
+  <int value="1" label="File closing error"/>
+  <int value="2" label="File creation error"/>
+  <int value="3" label="File writing error"/>
+  <int value="4" label="Frame no longer exists"/>
+  <int value="5" label="Main frame serialization forbidden"/>
+  <int value="6" label="Render process no longer exists"/>
+</enum>
+
 <enum name="MicrophoneMuteResult" type="int">
   <int value="0" label="Muted"/>
   <int value="1" label="Not muted"/>