Add some metrics for printing with extension

chrome/browser/ui/webui/print_preview/print_preview_handler.cc

Index: chrome/browser/ui/webui/print_preview/print_preview_handler.cc
diff --git a/chrome/browser/ui/webui/print_preview/print_preview_handler.cc b/chrome/browser/ui/webui/print_preview/print_preview_handler.cc
index 51b36bb40a3645c53958b8a021de67726cb6fb42..e465e03e7ff419aca129f7ffbebdab33252d3f00 100644
--- a/chrome/browser/ui/webui/print_preview/print_preview_handler.cc
+++ b/chrome/browser/ui/webui/print_preview/print_preview_handler.cc
@@ -107,6 +107,7 @@ enum UserActionBuckets {
   INITIATOR_CLOSED,
   PRINT_WITH_CLOUD_PRINT,
   PRINT_WITH_PRIVET,
+  PRINT_WITH_EXTENSION,
   USERACTION_BUCKET_BOUNDARY
 };
 
@@ -884,7 +885,9 @@ void PrintPreviewHandler::HandlePrint(const base::ListValue* args) {
 #endif
 
   if (print_with_extension) {
-    // TODO(tbarzic): Record UMA stats.
+    UMA_HISTOGRAM_COUNTS("PrintPreview.PageCount.PrintWithExtension",
+                         page_count);

[Ilya Sherman, 2015/02/21 00:26:39]

I bet that UMA_HISTOGRAM_COUNTS_100 or _10000 would be better for this
histogram, but I understand if you want to keep it inline with the existing
histograms.  (Note that if you want to change the existing histograms to use a
different macro, you should rename those histograms as well, and mark the old
names as <obsolete> in histograms.xml.)

+    ReportUserActionHistogram(PRINT_WITH_EXTENSION);
 
     std::string destination_id;
     std::string print_ticket;