Log the creation of a dialog boxes using DialogDelegate.

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 12b8f38552a570320bf2fdd9795435ae19855797..b7c7bbb89cebe8d685bc1ea18d4862beb74e5b8f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -11788,6 +11788,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Dialog.Delegate.Creation">

[jwd, 2017/05/03 20:14:40]

Add an enum. Looks like we have a BooleanCreated enum already, so you can use
that.

[pdyson, 2017/05/03 23:59:00]

Done.

+  <owner>pdyson@chromium.org</owner>
+  <summary>
+    Counts the number times dialog boxes are created using DialogDelegate.
+  </summary>
+</histogram>
+
 <histogram name="DirectWrite.Fonts.BuildCache.File.Size" units="KB">
   <owner>shrikant@chromium.org</owner>
   <summary>