Add UMA histogram to measure how long beforeunload dialogs are open.

This is part of a larger project to measure page load times starting
at user input. One blocker for this is understanding how the
beforeunload dialog might affect this metric.

Bug: 750639
Change-Id: I7aef267a52353a6852dd82bad6c6c74a6fb82e51
Reviewed-on: https://chromium-review.googlesource.com/1162673
Commit-Queue: Annie Sullivan <sullivan@chromium.org>
Reviewed-by: Shubhie Panicker <panicker@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582992}

third_party/blink/renderer/core/dom/document.cc

@@ -3538,8 +3538,16 @@ bool Document::DispatchBeforeUnloadEvent(ChromeClient& chrome_client,
   String text = before_unload_event.returnValue();
   beforeunload_dialog_histogram.Count(
       BeforeUnloadDialogHistogramEnum::kShowDialog);
-  if (chrome_client.OpenBeforeUnloadConfirmPanel(text, frame_, is_reload)) {
-    did_allow_navigation = true;
+  const TimeTicks beforeunload_confirmpanel_start = CurrentTimeTicks();
+  did_allow_navigation =
+      chrome_client.OpenBeforeUnloadConfirmPanel(text, frame_, is_reload);
+  const TimeTicks beforeunload_confirmpanel_end = CurrentTimeTicks();
+  if (did_allow_navigation) {
+    // Only record when a navigation occurs, since we want to understand
+    // the impact of the before unload dialog on overall input to navigation.
+    UMA_HISTOGRAM_MEDIUM_TIMES(
+        "DocumentEventTiming.BeforeUnloadDialogDuration.ByNavigation",
+        beforeunload_confirmpanel_end - beforeunload_confirmpanel_start);
     return true;
   }
 

tools/metrics/histograms/histograms.xml

@@ -19891,6 +19891,17 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="DocumentEventTiming.BeforeUnloadDialogDuration.ByNavigation"
+    units="ms" expires_after="2019-08-09">
+  <owner>sullivan@chromium.org</owner>
+  <summary>
+    Milliseconds spent in beforeunload dialog. Does not include time to process
+    beforeunload event in JavaScript (see
+    DocumentEventTiming.BeforeUnloadDuration). Recorded immediately after
+    closing dialog, upon user confirmation to navigate away from the page.
+  </summary>
+</histogram>
+
 <histogram name="DocumentEventTiming.BeforeUnloadDuration" units="microseconds">
   <owner>panicker@chromium.org</owner>
   <summary>