Histogram the scheme of an origin on the 1st navigation to it.

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 cb2565427d0c3ac9bfcb7235717f50202177feb7..446a40c837758d7609fa3103c3124d6d4a91b0e7 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -23188,6 +23188,26 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Navigation.IncognitoMainFrameScheme" enum="NavigationScheme">
+  <owner>palmer@chromium.org</owner>
+  <owner>felt@chromium.org</owner>
+  <summary>
+    The scheme of the URL for the first main-frame navigation per origin per
+    Incognito session.

[felt, 2016/04/01 00:01:59]

It seems a bit misleading to call it MFS, when it is once per visit. Could you
rename it? Also, would it make sense to have the similar policy in place for
Non-Incognito for comparison?

[palmer, 2016/04/05 23:42:18]

Sure, will do. Proposed name: Navigation.SchemePerUniqueOrigin[OTR].

+  </summary>
+</histogram>
+
+<histogram name="Navigation.IncognitoMainFrameSchemeDifferentPage"

[felt, 2016/04/01 00:01:59]

Why do we need both of these, if it's already limited to once per visit?

[palmer, 2016/04/05 23:42:18]

Hmm, yeah. I'll remove it.

+    enum="NavigationScheme">
+  <owner>palmer@chromium.org</owner>
+  <owner>felt@chromium.org</owner>
+  <summary>
+    The scheme of the URL for the first main-frame navigation that goes to a
+    different page per origin per Incognito session. Transitions like fragment
+    change or history.pushState are excluded.
+  </summary>
+</histogram>
+
 <histogram name="Navigation.IsMobileOptimized" enum="BooleanIsMobileOptimized">
   <owner>cjhopman@chromium.org</owner>
   <owner>nyquist@chromium.org</owner>