[ios] Move page load count to use WebStateObserver::DidStartNavigation.

Currently page load count is done via the call to
StabilityMetricsHelper::LogLoadStarted in WebStateDidStartLoading.
DidStartLoading was never an ideal location, and the move to
WKBasedNavigationManager introduces even more complexity.

Instead, move this call to WebStateDidStartNavigation.

Rollout will start behind a feature flag that will be paired with the
SlimNav experiment and control groups.

Bug: 786547
Change-Id: I1100bd0cca68fe511b992c1df67c449a6739fc4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1632570Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664016}

ios/chrome/browser/metrics/BUILD.gn

@@ -18,6 +18,16 @@ source_set("ukm_url_recorder") {
   ]
 }
 
+source_set("features") {
+  sources = [
+    "features.cc",
+    "features.h",
+  ]
+  deps = [
+    "//base",
+  ]
+}
+
 source_set("metrics") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
@@ -47,6 +57,7 @@ source_set("metrics") {
     ":ukm_url_recorder",
   ]
   deps = [
+    ":features",
     "//base",
     "//components/browser_sync",
     "//components/crash/core/common",

ios/chrome/browser/metrics/features.cc

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "ios/chrome/browser/metrics/features.h"
+
+const base::Feature kLogLoadStartedInDidStartNavigation{
+    "LogLoadStartedInDidStartNavigation", base::FEATURE_DISABLED_BY_DEFAULT};

ios/chrome/browser/metrics/features.h

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef IOS_CHROME_BROWSER_METRICS_FEATURES_H_
+#define IOS_CHROME_BROWSER_METRICS_FEATURES_H_
+
+#include "base/feature_list.h"
+
+// Feature flag to move -LogLoadStarted() to WebStateDidStartNavigation().
+extern const base::Feature kLogLoadStartedInDidStartNavigation;
+
+#endif  // IOS_CHROME_BROWSER_METRICS_FEATURES_H_

ios/chrome/browser/metrics/ios_chrome_stability_metrics_provider.mm

@@ -4,8 +4,10 @@
 
 #include "ios/chrome/browser/metrics/ios_chrome_stability_metrics_provider.h"
 
+#include "base/feature_list.h"
 #include "base/metrics/histogram_macros.h"
 #include "ios/chrome/browser/chrome_url_constants.h"
+#include "ios/chrome/browser/metrics/features.h"
 #import "ios/web/public/web_state/navigation_context.h"
 #import "ios/web/public/web_state/web_state.h"
 
@@ -63,7 +65,8 @@ void IOSChromeStabilityMetricsProvider::WebStateDidStartLoading(
     return;
 
   UMA_HISTOGRAM_BOOLEAN(kPageLoadCountLoadingStartedMetric, true);
-  helper_.LogLoadStarted();
+  if (!base::FeatureList::IsEnabled(kLogLoadStartedInDidStartNavigation))
+    helper_.LogLoadStarted();
 }
 
 void IOSChromeStabilityMetricsProvider::WebStateDidStartNavigation(
@@ -79,7 +82,8 @@ void IOSChromeStabilityMetricsProvider::WebStateDidStartNavigation(
   } else if (navigation_context->IsSameDocument()) {
     type = PageLoadCountNavigationType::SAME_DOCUMENT_WEB_NAVIGATION;
   } else {
-    // TODO(crbug.com/786547): Move helper_.LogLoadStarted() here.
+    if (base::FeatureList::IsEnabled(kLogLoadStartedInDidStartNavigation))
+      helper_.LogLoadStarted();
   }
   UMA_HISTOGRAM_ENUMERATION(kPageLoadCountMetric, type,
                             PageLoadCountNavigationType::COUNT);