Document iOS "Session.*" histograms.

The histograms "Session.ClosedTabCounts", "Session.NewTabCounts" and
"Session.OpenedTabCounts" have never been documented and cannot be
easily be checked on uma. Document them to fix this.

BUG=None

Review-Url: https://codereview.chromium.org/2671993003
Cr-Commit-Position: refs/heads/master@{#448010}

tools/metrics/histograms/histograms.xml

@@ -61373,6 +61373,38 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Session.ClosedTabCounts" units="operations">
+  <owner>rohitrao@chromium.org</owner>
+  <summary>
+    The number of tabs closed by the user during the current session
+    (launch/foregrounding to backgrounding).
+
+    This histogram is only logged on iOS when the application is backgrounded.
+  </summary>
+</histogram>
+
+<histogram name="Session.NewTabCounts" units="operations">
+  <owner>rohitrao@chromium.org</owner>
+  <summary>
+    The number of tabs created by the user during the current session
+    (launch/foregrounding to backgrounding).
+
+    This histogram is only logged on iOS when the application is backgrounded.
+  </summary>
+</histogram>
+
+<histogram name="Session.OpenedTabCounts" units="operations">
+  <owner>rohitrao@chromium.org</owner>
+  <summary>
+    The number of times the user changed the active tab (this can happen when
+    manually selecting a new tab, when a pre-rendered tab is made active, when
+    closing the current tab and the next one is selected) during the current
+    session (launch/foregrounding to backgrounding).
+
+    This histogram is only logged on iOS when the application is backgrounded.
+  </summary>
+</histogram>
+
 <histogram name="Session.TotalDuration" units="ms">
   <owner>mariakhomenko@chromium.org</owner>
   <owner>fqian@chromium.org</owner>