Add UMA metrics to see the proportions of dialogs spawned by foremost pages.

chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc

Index: chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
diff --git a/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc b/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
index dd52e788c6c634380fed2c5ca9a9966a4076e6e1..5d69513500f18b3ac35565dc606ee2a86e1f069d 100644
--- a/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
+++ b/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
@@ -76,10 +76,35 @@ void JavaScriptDialogTabHelper::RunJavaScriptDialog(
                          message_length);
   }
 
-  if (IsEnabled()) {
-    content::WebContents* parent_web_contents =
-        WebContentsObserver::web_contents();
+  content::WebContents* parent_web_contents =
+      WebContentsObserver::web_contents();
+  enum class Foremost {
+    FOREMOST = 0,
+    NOT_FOREMOST = 1,
+    MAX,
+  };
+  Foremost foremost = IsWebContentsForemost(parent_web_contents)
+                          ? Foremost::FOREMOST
+                          : Foremost::NOT_FOREMOST;
+  switch (message_type) {
+    case content::JAVASCRIPT_MESSAGE_TYPE_ALERT:
+      UMA_HISTOGRAM_ENUMERATION("JSDialogs.ForemostStatus.Alert",

[Rick Byers, 2016/11/13 23:36:27]

nit: I'd find this simpler to read if you had only one UMA_HISTOGRAM_ENUMERATION
call and just make the histogram name conditional.  But I'm OK either way.

[Rick Byers, 2016/11/13 23:36:27]

Note that you could use UMA_HISTOGRAM_BOOLEAN for this.  I don't know if there's
any advantage - your histograms.xml reviewer may have an opinion.

[Avi (use Gerrit), 2016/11/13 23:52:56]

Right....

Every time I do histograms I learn something I then forget. This was swiped from
another two-value enumeration that was less clearly true/false, so the reviewer
then preferred a two-valued enum.

"is foremost" is very boolean, so I'll switch.

+                                static_cast<int>(foremost),
+                                static_cast<int>(Foremost::MAX));
+      break;
+    case content::JAVASCRIPT_MESSAGE_TYPE_CONFIRM:
+      UMA_HISTOGRAM_ENUMERATION("JSDialogs.ForemostStatus.Confirm",
+                                static_cast<int>(foremost),
+                                static_cast<int>(Foremost::MAX));
+      break;
+    case content::JAVASCRIPT_MESSAGE_TYPE_PROMPT:
+      UMA_HISTOGRAM_ENUMERATION("JSDialogs.ForemostStatus.Prompt",
+                                static_cast<int>(foremost),
+                                static_cast<int>(Foremost::MAX));
+      break;
+  }
 
+  if (IsEnabled()) {
     if (!IsWebContentsForemost(parent_web_contents) &&
         message_type == content::JAVASCRIPT_MESSAGE_TYPE_PROMPT) {
       // Don't allow "prompt" dialogs to steal the user's focus. TODO(avi):