Add histograms to record document.execCommand.

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 df25d22f7d4b95c15a19b023300db6d10448dba9..9385070d4b986777f1c8ba0edbca6f5ea9aa9b97 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -29665,7 +29665,7 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
-<histogram name="WebCore.Editing.Commands" enum="MappedEditingCommands">
+<histogram name="WebCore.Document.execCommand" enum="MappedEditingCommands">
   <owner>yoichio@chromium.org</owner>
   <summary>
     Counts the number of times each document.execCommand is executed. This
@@ -29673,6 +29673,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="WebCore.Editing.Commands" enum="MappedEditingCommands">
+  <owner>yoichio@chromium.org</owner>
+  <summary>
+    Counts the number of times each Editor::Command::execute is called. This
+    doesn't count commands not supported by Blink.
+  </summary>
+</histogram>
+
 <histogram name="WebCore.FeatureObserver" enum="FeatureObserver">
   <owner>eseidel@chromium.org</owner>
   <summary>