Add Incognito Navigation histograms

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 a17bd3d33d7a34a69f15a94de0452dca70a07f92..c0d85f9a3d78f598bb297c18d8f0405a22ef6d04 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -31583,6 +31583,25 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Navigation.MainFrameSchemeDifferentPageOTR"
+    enum="NavigationScheme">
+  <owner>cbentzel@chromium.org</owner>
+  <owner>davidben@chromium.org</owner>

[davidben, 2017/04/20 19:26:28]

Should you all be owning these histograms? It seems you're the main users of
these anyway. Chris and I just took ownership of the old ones because they were
unowned.

+  <summary>
+    The scheme of the URL for each main-frame navigation that goes to a
+    different page while in incognito. Transitions like fragment change or
+    history.pushState are excluded.

[Mark P, 2017/04/19 22:45:45]

Are they included in the one below?

(If yes, no change is necessary.)

[elawrence, 2017/04/19 22:55:13]

Yes. The difference between MainFrameSchemeDifferentPage*{OTR} and
MainFrameScheme{OTR} is that the "DifferentPage" version of the metric is only
recorded if the navigation goes to a different page; that is to say, when it is
NOT a fragment change or history.pushState.

By subtracting MainFrameSchemeDifferentPage from MainFrameScheme, one could
calculate the number of intra-page navigations.

+  </summary>
+</histogram>
+
+<histogram name="Navigation.MainFrameSchemeOTR" enum="NavigationScheme">
+  <owner>cbentzel@chromium.org</owner>
+  <owner>davidben@chromium.org</owner>
+  <summary>The scheme of the URL for each main-frame navigation while in
+    incognito.
+  </summary>
+</histogram>
+
 <histogram name="Navigation.OnBeforeUnloadOverheadTime" units="ms">
   <owner>carlosk@chromium.org</owner>
   <summary>