[page_load_metrics] Add a NavigationThrottle for richer abort metrics

chrome/browser/page_load_metrics/metrics_navigation_throttle.cc

+// Copyright 2016 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 "chrome/browser/page_load_metrics/metrics_navigation_throttle.h"
+
+#include "base/memory/ptr_util.h"
+#include "chrome/browser/page_load_metrics/metrics_web_contents_observer.h"
+#include "content/public/browser/navigation_handle.h"
+
+namespace page_load_metrics {
+
+// static
+std::unique_ptr<content::NavigationThrottle> MetricsNavigationThrottle::Create(
+    content::NavigationHandle* handle) {
+  return base::WrapUnique(new MetricsNavigationThrottle(handle));
+}
+
+MetricsNavigationThrottle::~MetricsNavigationThrottle() {}
+
+content::NavigationThrottle::ThrottleCheckResult
+MetricsNavigationThrottle::WillStartRequest() {
+  MetricsWebContentsObserver* observer =
+      MetricsWebContentsObserver::FromWebContents(
+          navigation_handle()->GetWebContents());
+  if (observer)
+    observer->WillStartNavigationRequest(navigation_handle());
+  return content::NavigationThrottle::PROCEED;
+}
+
+MetricsNavigationThrottle::MetricsNavigationThrottle(
+    content::NavigationHandle* handle)
+    : content::NavigationThrottle(handle) {}
+
+}  // namespace page_load_metrics