[page_load_metrics] Add a NavigationThrottle for richer abort metrics

components/page_load_metrics/browser/metrics_navigation_throttle.h

Index: components/page_load_metrics/browser/metrics_navigation_throttle.h
diff --git a/components/page_load_metrics/browser/metrics_navigation_throttle.h b/components/page_load_metrics/browser/metrics_navigation_throttle.h
new file mode 100644
index 0000000000000000000000000000000000000000..1f39b7947cc5436f9753a64ac2e867469780b213
--- /dev/null
+++ b/components/page_load_metrics/browser/metrics_navigation_throttle.h
@@ -0,0 +1,36 @@
+// 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.
+
+#ifndef COMPONENTS_PAGE_LOAD_METRICS_BROWSER_METRICS_NAVIGATION_THROTTLE_H_
+#define COMPONENTS_PAGE_LOAD_METRICS_BROWSER_METRICS_NAVIGATION_THROTTLE_H_
+
+#include <memory>
+
+#include "base/macros.h"
+#include "content/public/browser/navigation_throttle.h"
+
+namespace page_load_metrics {
+
+// This class is used to forward calls to the MetricsWebContentsObserver.
+// Namely, WillStartRequest() is called on NavigationThrottles, but not on
+// WebContentsObservers. Data from the NavigationHandle accessed at this point
+// is used to obtain more reliable abort metrics (like page transition type).
+class MetricsNavigationThrottle : public content::NavigationThrottle {
+ public:
+  static std::unique_ptr<content::NavigationThrottle> Create(
+      content::NavigationHandle* handle);
+  ~MetricsNavigationThrottle() override;
+
+  // content::NavigationThrottle:
+  content::NavigationThrottle::ThrottleCheckResult WillStartRequest() override;
+
+ private:
+  explicit MetricsNavigationThrottle(content::NavigationHandle* handle);
+
+  DISALLOW_COPY_AND_ASSIGN(MetricsNavigationThrottle);
+};
+
+}  // namespace page_load_metrics
+
+#endif  // COMPONENTS_PAGE_LOAD_METRICS_BROWSER_METRICS_NAVIGATION_THROTTLE_H_