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;
+
+// Tracks user input events from web contents and notifies
+// |DesktopEngagementService|.
+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_