Add Profile.Incognito.MainFrameNavigationsPerSession to histograms.xml

"Profile.Incognito.MainFrameNavigationsPerSession" metric was added in
crrev.com/c/2421821 but histograms.xml was not updated.

Bug: 966747
Change-Id: Iae36289df5532a5c07924f794632817133c4eefb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2450316
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814182}

tools/metrics/histograms/histograms_xml/profile/histograms.xml

@@ -199,6 +199,17 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="Profile.Incognito.MainFrameNavigationsPerSession"
+    units="navigations" expires_after="2021-06-01">
+  <owner>rhalavati@chromium.org</owner>
+  <owner>chrome-privacy-core@google.com</owner>
+  <summary>
+    This histogram records the number of mainframe navigations that have been
+    done during one Incognito session. It is recorded once an off the record
+    profile for an incognito session is closed.
+  </summary>
+</histogram>
+
 <histogram name="Profile.Menu.ClickedActionableItem"
     enum="ProfileMenuActionableItem" expires_after="2021-06-30">
   <owner>droger@chromium.org</owner>