Adding UMA stats for dragdrop events for both touch and mouse.

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 f879a89c876967194b6b5b4b2422d07978306e67..e4dca7ca3fb9658ba6a0d596541f2bfc96fa9275 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -6089,6 +6089,35 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <summary>The write size for calls to BaseFile::AppendDataTofile().</summary>
 </histogram>
 
+<histogram name="DragDrop.Cancel" enum="DragDropEventSource">

[Alexei Svitkine (slow), 2015/01/22 20:02:20]

This seems to be adding a new top-level prefix. Can it be nested under an
existing one? For example, looks like these are specific to Ash, so how about
Ash.DragDrop.Cancel and such?

[caelyn, 2015/01/22 21:13:08]

These are not Ash specific. How about Event.DragDrop.Cancel and such?

[Alexei Svitkine (slow), 2015/01/22 21:32:55]

Event.* SGTM

+  <owner>mfomitchev@chromium.org</owner>
+  <summary>
+    Counts the number of times the user cancelled a drag and drop operation.
+  </summary>
+</histogram>
+
+<histogram name="DragDrop.Drop" enum="DragDropEventSource">
+  <owner>mfomitchev@chromium.org</owner>
+  <summary>
+    Counts the number of times the user completed a drag and drop operation.
+  </summary>
+</histogram>
+
+<histogram name="DragDrop.ExternalOriginDrop">
+  <owner>mfomitchev@chromium.org</owner>
+  <summary>
+    Counts the number of times a drag and drop operation originating outside of
+    a chrome window successfuly drops.

[Alexei Svitkine (slow), 2015/01/22 20:02:20]

Nit: capitalize Chrome

[caelyn, 2015/01/22 21:13:08]

Done.

+  </summary>
+</histogram>
+
+<histogram name="DragDrop.Start" enum="DragDropEventSource">
+  <owner>mfomitchev@chromium.org</owner>
+  <summary>
+    Counts the number of times the user started a drag and drop operation.
+  </summary>
+</histogram>
+
 <histogram name="Drive.CacheDBOpenStatus" enum="DriveCacheDBOpenStatus">
   <obsolete>
     Deprecated 8/2013.
@@ -44646,6 +44675,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="4" label="Initiated by Resumption"/>
 </enum>
 
+<enum name="DragDropEventSource" type="int">
+  <int value="0" label="Mouse"/>
+  <int value="1" label="Touch"/>
+</enum>
+
 <enum name="DriveCacheDBOpenStatus" type="int">
   <int value="0" label="Success"/>
   <int value="1" label="Corrupt database"/>