NTP: Add histogram measuring the main bundle's load time.

Specifically adding a new NewTabPage.MainUi.ShownTime histogram.

Fixed: 1141327
Change-Id: Ide414b94c8273e19448ef8251bcede671792ef82
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533643Reviewed-by: Alex Gough <ajgo@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Alex Gough <ajgo@chromium.org>
Commit-Queue: dpapad <dpapad@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826952}

chrome/browser/resources/new_tab_page/app.js

@@ -351,6 +351,7 @@ class AppElement extends PolymerElement {
   /** @override */
   ready() {
     super.ready();
+    this.pageHandler_.onAppRendered(BrowserProxy.getInstance().now());
     // Let the browser breath and then render remaining elements.
     BrowserProxy.getInstance().waitForLazyRender().then(() => {
       ensureLazyLoaded();

chrome/browser/ui/search/ntp_user_data_logger.cc

@@ -552,6 +552,9 @@ void NTPUserDataLogger::LogEvent(NTPLoggingEventType event,
     case NTP_MODULES_SHOWN:
       UMA_HISTOGRAM_LOAD_TIME("NewTabPage.Modules.ShownTime", time);
       break;
+    case NTP_APP_RENDERED:
+      UMA_HISTOGRAM_LOAD_TIME("NewTabPage.MainUi.ShownTime", time);
+      break;
   }
 }
 

chrome/browser/ui/webui/new_tab_page/new_tab_page.mojom

@@ -403,6 +403,8 @@ interface PageHandler {
   OnModuleUsage(string module_id);
   // Logs that the modules have been shown at |time|.
   OnModulesRendered(double time);
+  // Logs that the <ntp-app> element's |ready| callback method was called.
+  OnAppRendered(double time);
 
   // ======= REALBOX =======
   // Queries autocomplete matches from the browser.

chrome/browser/ui/webui/new_tab_page/new_tab_page_handler.cc

@@ -728,6 +728,11 @@ void NewTabPageHandler::OnPromoServiceShuttingDown() {
   promo_service_ = nullptr;
 }
 
+void NewTabPageHandler::OnAppRendered(double time) {
+  logger_->LogEvent(NTP_APP_RENDERED,
+                    base::Time::FromJsTime(time) - ntp_navigation_start_time_);
+}
+
 void NewTabPageHandler::OnMostVisitedTilesRendered(
     std::vector<new_tab_page::mojom::MostVisitedTilePtr> tiles,
     double time) {

chrome/browser/ui/webui/new_tab_page/new_tab_page_handler.h

@@ -109,6 +109,7 @@ class NewTabPageHandler : public new_tab_page::mojom::PageHandler,
   void OnRestoreModule(const std::string& module_id) override;
   void SetModulesVisible(bool visible) override;
   void UpdateModulesVisible() override;
+  void OnAppRendered(double time) override;
   void OnMostVisitedTilesRendered(
       std::vector<new_tab_page::mojom::MostVisitedTilePtr> tiles,
       double time) override;

chrome/common/search/ntp_logging_events.h

@@ -176,7 +176,10 @@ enum NTPLoggingEventType {
   // The NTP modules were shown.
   NTP_MODULES_SHOWN = 83,
 
-  NTP_EVENT_TYPE_LAST = NTP_MODULES_SHOWN
+  // The NTP <ntp-app> element was created and ready() was called.
+  NTP_APP_RENDERED = 84,
+
+  NTP_EVENT_TYPE_LAST = NTP_APP_RENDERED
 };
 
 // The different types of events that are logged for NTP search suggestions,

tools/metrics/histograms/histograms_xml/new_tab_page/histograms.xml

@@ -737,6 +737,17 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="NewTabPage.MainUi.ShownTime" units="ms"
+    expires_after="2021-11-12">
+  <owner>tiborg@chromium.org</owner>
+  <owner>chrome-desktop-ntp@google.com</owner>
+  <summary>
+    Records the time until the top-level ntp-app Custom Element's |ready|
+    callback is called. The intention is to measure how quickly the contents of
+    new_tab_page.rollup.js (main JS bundle) are rendered.
+  </summary>
+</histogram>
+
 <histogram name="NewTabPage.MobileIsUserOnline" enum="Boolean"
     expires_after="M85">
   <owner>fserb@chromium.org</owner>