Add histograms for observing UMA throttling effect.

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 cbbbc4c4c7cc6a7821f8fb7e48022fab20963287..b6364e5fc86ca60a60c61a2248d382dccae4f4e9 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -54669,6 +54669,10 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram name="UMA.ActualLogUploadInterval" units="minutes">
+  <obsolete>
+    Used for analyzing UMA log uploads on cellular connection, but necessary
+    after the analysis is finished.
+  </obsolete>
   <owner>asvitkine@chromium.org</owner>
   <summary>
     The actual interval between log upload start and previous log upload
@@ -54855,7 +54859,20 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="UMA.LogUpload.Canceled.CellularConstraint">
+  <owner>asvitkine@chromium.org</owner>
+  <owner>gayane@chromium.org</owner>
+  <summary>
+    Simple counter of the log upload cancel events due to throttling on

[Alexei Svitkine (slow), 2016/04/08 20:07:13]

I would make the description more clear. What's a "log upload cancel event"?

A more clear description could be: 

Logs whether a log was not uploaded due to cellular log throttling logic.
Android only.

[gayane -on leave until 09-2017, 2016/04/08 20:40:14]

Done.

+    cellular.
+  </summary>
+</histogram>
+
 <histogram name="UMA.LogUpload.ConnetionType" enum="NetworkConnectionType">
+  <obsolete>
+    Used for analyzing UMA log uploads on cellular connection, but necessary
+    after the analysis is finished.
+  </obsolete>
   <owner>asvitkine@chromium.org</owner>
   <owner>gayane@chromium.org</owner>
   <summary>
@@ -55052,6 +55069,12 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </details>
 </histogram>
 
+<histogram name="UMA.UnSentLog.Dropped">
+  <owner>asvitkine@chromium.org</owner>
+  <owner>gayane@chromium.org</owner>
+  <summary>Counter for number of UMA unsent logs dropped.</summary>

[Alexei Svitkine (slow), 2016/04/08 20:07:13]

Please explain what "dropped" means.

[gayane -on leave until 09-2017, 2016/04/08 20:40:14]

Done.

+</histogram>
+
 <histogram name="UMA.UploadCreation" enum="BooleanSuccess">
   <obsolete>
     Deprecated as of August 2015.  This failure case no longer exists.