Add desktop engagement metrics.

chrome/browser/metrics/desktop_engagement/desktop_engagement_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_METRICS_DESKTOP_ENGAGEMENT_DESKTOP_ENGAGEMENT_OBSERVER_H_
+#define CHROME_BROWSER_METRICS_DESKTOP_ENGAGEMENT_DESKTOP_ENGAGEMENT_OBSERVER_H_
+
+#include "base/macros.h"
+#include "content/public/browser/render_widget_host.h"
+#include "content/public/browser/web_contents.h"
+#include "content/public/browser/web_contents_observer.h"
+#include "content/public/browser/web_contents_user_data.h"
+#include "third_party/WebKit/public/web/WebInputEvent.h"
+
+namespace metrics {
+
+class DesktopEngagementService;
+
+// Class for tracking user interaction with the browser.

[Alexei Svitkine (slow), 2016/07/28 18:37:22]

Please make explain this more.

You can omit "Class for" since that doesn't add anything. Mention that this is
used to monitor input events from web contents and notify the desktop engagement
service.

[gayane -on leave until 09-2017, 2016/07/28 21:21:34]

Done.

+class DesktopEngagementObserver
+    : public content::WebContentsObserver,
+      public content::WebContentsUserData<DesktopEngagementObserver>,
+      public content::RenderWidgetHost::InputEventObserver {
+ public:
+  DesktopEngagementObserver(content::WebContents* web_contents,
+                            DesktopEngagementService* service);
+  ~DesktopEngagementObserver() override;
+
+  static DesktopEngagementObserver* CreateForWebContents(
+      content::WebContents* web_contents);
+
+ private:
+  friend class content::WebContentsUserData<DesktopEngagementObserver>;
+
+  // Register / Unregister input event callback to given RenderViewHost
+  void RegisterInputEventObserver(content::RenderViewHost* host);
+  void UnregisterInputEventObserver(content::RenderViewHost* host);
+
+  // content::RenderWidgetHost::InputEventObserver:
+  void OnInputEvent(const blink::WebInputEvent& event) override;
+
+  // content::WebContentsObserver:
+  void RenderViewHostChanged(content::RenderViewHost* old_host,
+                             content::RenderViewHost* new_host) override;
+
+  DesktopEngagementService* service_;
+
+  DISALLOW_COPY_AND_ASSIGN(DesktopEngagementObserver);
+};
+
+}  // namespace metrics
+
+#endif  // CHROME_BROWSER_METRICS_DESKTOP_ENGAGEMENT_DESKTOP_ENGAGEMENT_OBSERVER_H_