Refactor code listening to platform events in content/renderer/.

content/renderer/screen_orientation/screen_orientation_observer.h

+// Copyright 2014 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 "content/public/renderer/platform_event_observer.h"
+
+namespace content {
+
+// ScreenOrientationObserver is a helper class implementing the
+// PlatformEventObserver template with blink::WebPlatformEventListener, which is
+// a pure virtual class because it doesn't expect listeners.
+// It is implemented on top of PlatformEventObserver to make sure it follows a
+// common code path even though it only uses part of this code path: it is not
+// expected to receive messages back but should send messages to start/stop
+// listening.
+// The intent of this class is to make sure that platforms that can't listen for
+// display rotations at a marginal cost can be told when to actually do it.
+class ScreenOrientationObserver
+    : public PlatformEventObserver<blink::WebPlatformEventListener> {
+ public:
+  ScreenOrientationObserver();
+
+  // Overriding this method just to make sure |listener| is always null.
+  virtual void Start(blink::WebPlatformEventListener* listener) OVERRIDE;
+
+ protected:
+  virtual void SendStartMessage() OVERRIDE;
+  virtual void SendStopMessage() OVERRIDE;
+};
+
+}; // namespace content