Add Printing.CUPS.PrintersDiscovered to our metrics.

chrome/browser/ui/webui/settings/chromeos/cups_printers_handler.cc

Index: chrome/browser/ui/webui/settings/chromeos/cups_printers_handler.cc
diff --git a/chrome/browser/ui/webui/settings/chromeos/cups_printers_handler.cc b/chrome/browser/ui/webui/settings/chromeos/cups_printers_handler.cc
index 28a1781f5cad216bd1123cd274008f7d44ca2c0e..544005640147e0f46b4e3338220f0ab2fa84cb19 100644
--- a/chrome/browser/ui/webui/settings/chromeos/cups_printers_handler.cc
+++ b/chrome/browser/ui/webui/settings/chromeos/cups_printers_handler.cc
@@ -11,6 +11,7 @@
 #include "base/files/file_util.h"
 #include "base/json/json_string_value_serializer.h"
 #include "base/memory/ptr_util.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/path_service.h"
 #include "base/strings/string_util.h"
 #include "base/threading/sequenced_task_runner_handle.h"
@@ -400,6 +401,7 @@ void CupsPrintersHandler::HandleStopDiscovery(const base::ListValue* args) {
 }
 
 void CupsPrintersHandler::OnPrintersFound(
+    bool scan_done,
     const std::vector<Printer>& printers) {
   std::unique_ptr<base::ListValue> printers_list =
       base::MakeUnique<base::ListValue>();
@@ -408,10 +410,12 @@ void CupsPrintersHandler::OnPrintersFound(
   }
 
   FireWebUIListener("on-printer-discovered", *printers_list);
-}
 
-void CupsPrintersHandler::OnDiscoveryInitialScanDone() {

[dpapad, 2017/05/26 21:02:03]

Is it necessary to merge OnDiscoveryInitialScanDone to OnPrintersFound? Can we
avoid merging them (and the need for the  |scan_done| boolean) and adding a
boolean to OnPrintersFound by doing the following instead?

1) Keep the 2 observer methods separate.
2) Pass printers_list->GetSize() as a parameter to OnDiscoveryInitialScanDone.
3) Call UMA_HISTOGRAM_COUNTS_100() from OnDiscoveryInitialScanDone.

[skau, 2017/05/26 21:20:10]

Justin and I have been considering whether the current change or your proposed
alternative would be simpler.  We think that the change as written is a more
straightforward API independent of adding metrics.  All
OnDiscoveryInitialScanDone calls will be immediately preceded by an
OnPrintersFound call and we haven't found it useful to have a separate method.

-  FireWebUIListener("on-printer-discovery-done");
+  if (scan_done) {
+    UMA_HISTOGRAM_COUNTS_100("Printing.CUPS.PrintersDiscovered",
+                             printers_list->GetSize());
+    FireWebUIListener("on-printer-discovery-done");
+  }
 }
 
 }  // namespace settings