Add UMA histograms to track very brief or frequent tabs and JS dialogs.

components/app_modal/javascript_dialog_manager.cc

Index: components/app_modal/javascript_dialog_manager.cc
diff --git a/components/app_modal/javascript_dialog_manager.cc b/components/app_modal/javascript_dialog_manager.cc
index 2b6ec646cebf8d6edc574a13e5f4bb43a38df1c7..c9fd1a0d166eb12a6fd4cf49c7503d7f958eedf3 100644
--- a/components/app_modal/javascript_dialog_manager.cc
+++ b/components/app_modal/javascript_dialog_manager.cc
@@ -9,6 +9,7 @@
 #include "base/bind.h"
 #include "base/i18n/rtl.h"
 #include "base/macros.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/strings/utf_string_conversions.h"
 #include "components/app_modal/app_modal_dialog.h"
 #include "components/app_modal/app_modal_dialog_queue.h"
@@ -100,6 +101,25 @@ void JavaScriptDialogManager::RunJavaScriptDialog(
     return;
   }
 
+  base::TimeTicks now = base::TimeTicks::Now();
+  if (!last_creation_time_.is_null()) {
+    // A new dialog has been created: log the time since the last one was
+    // created.
+    UMA_HISTOGRAM_MEDIUM_TIMES(
+        "JSDialogs.FineTiming.TimeBetweenDialogCreatedAndNextDialogCreated",
+        now - last_creation_time_);
+  }
+  last_creation_time_ = now;
+
+  // Also log the time since a dialog was closed, but only if this is the first
+  // dialog that was opened since the closing.
+  if (!last_close_time_.is_null()) {
+    UMA_HISTOGRAM_MEDIUM_TIMES(
+        "JSDialogs.FineTiming.TimeBetweenDialogClosedAndNextDialogCreated",
+        now - last_close_time_);
+    last_close_time_ = base::TimeTicks();
+  }
+
   bool is_alert = message_type == content::JAVASCRIPT_MESSAGE_TYPE_ALERT;
   base::string16 dialog_title =
       GetTitle(web_contents, origin_url, accept_lang, is_alert);
@@ -249,6 +269,8 @@ void JavaScriptDialogManager::OnDialogClosed(
   // their WebContents is destroyed so |web_contents| is still valid here.)
   extensions_client_->OnDialogClosed(web_contents);
 
+  last_close_time_ = base::TimeTicks::Now();
+
   callback.Run(success, user_input);
 }