[Passwords] Log PasswordGeneration.PopupShown to UMA

This change adds logging for PasswordGeneration.PopupShown to UMA,
replicating the existing UKM metric.

Bug: 911846
Change-Id: I25556519d87927e63f9a37f923af1e01d717ec12
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521511Reviewed-by: Maria Kazinova <kazinova@google.com>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825186}

components/password_manager/core/browser/password_form_metrics_recorder.cc

@@ -281,6 +281,8 @@ PasswordFormMetricsRecorder::~PasswordFormMetricsRecorder() {
 
   if (password_generation_popup_shown_ !=
       PasswordGenerationPopupShown::kNotShown) {
+    UMA_HISTOGRAM_ENUMERATION("PasswordGeneration.PopupShown",
+                              password_generation_popup_shown_);
     ukm_entry_builder_.SetGeneration_PopupShown(
         static_cast<int64_t>(password_generation_popup_shown_));
   }

components/password_manager/core/browser/password_form_metrics_recorder.h

@@ -170,6 +170,7 @@ class PasswordFormMetricsRecorder
     kNotShown = 0,
     kShownAutomatically = 1,
     kShownManually = 2,
+    kMaxValue = kShownManually,
   };
 
   // Metric: PasswordGeneration.UserDecision

tools/metrics/histograms/histograms_xml/password/histograms.xml

@@ -83,6 +83,13 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="PasswordGeneration.PopupShown"
+    enum="PasswordGenerationPopupShown" expires_after="M92">
+  <owner>jdoerrie@chromium.org</owner>
+  <owner>kazinova@google.com</owner>
+  <summary>Records an entry if (and only if) a popup was shown.</summary>
+</histogram>
+
 <histogram name="PasswordGeneration.SubmissionAvailableEvent"
     enum="PasswordSubmissionEvent" expires_after="M90">
   <owner>jdoerrie@chromium.org</owner>