[page_load_metrics] Add support for chrome:// urls and add extensions observer

chrome/browser/page_load_metrics/observers/webui_page_load_metrics_observer.h

+// 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 CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_WEBUI_PAGE_LOAD_METRICS_OBSERVER_H_
+#define CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_WEBUI_PAGE_LOAD_METRICS_OBSERVER_H_
+
+#include "base/macros.h"
+#include "chrome/browser/page_load_metrics/page_load_metrics_observer.h"
+
+namespace internal {
+
+// Exposed for testing.
+extern const char kHistogramExtensionsParseDuration[];
+
+}  // namespace internal
+
+class WebUIPageLoadMetricsObserver
+    : public page_load_metrics::PageLoadMetricsObserver {
+ public:
+  WebUIPageLoadMetricsObserver();
+
+  // page_load_metrics::PageLoadMetricsObserver:
+  page_load_metrics::TrackingPolicy ShouldTrackLoad(
+      content::NavigationHandle* navigation_handle) override;
+  void OnParseStop(const page_load_metrics::PageLoadTiming& timing,
+                   const page_load_metrics::PageLoadExtraInfo& info) override;
+  void OnFirstContentfulPaint(
+      const page_load_metrics::PageLoadTiming& timing,
+      const page_load_metrics::PageLoadExtraInfo& info) override;
+
+ private:
+  enum TrackedPage {
+    NONE,
+    EXTENSIONS,
+  };
+  TrackedPage tracked_page_ = NONE;
+  DISALLOW_COPY_AND_ASSIGN(WebUIPageLoadMetricsObserver);
+};
+
+#endif  // CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_WEBUI_PAGE_LOAD_METRICS_OBSERVER_H_