Refactor: Make MediaCaptureDevicesDispatcher have pluggable handlers.

chrome/browser/media/tab_capture_access_handler.h

+// Copyright 2015 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_MEDIA_TAB_CAPTURE_ACCESS_HANDLER_H_
+#define CHROME_BROWSER_MEDIA_TAB_CAPTURE_ACCESS_HANDLER_H_
+
+#include "chrome/browser/media/media_access_handler.h"
+
+class MediaStreamCaptureIndicator;
+
+class TabCaptureAccessHandler : public MediaAccessHandler {
+ public:
+  TabCaptureAccessHandler();
+  ~TabCaptureAccessHandler() override;
+
+  // MediaAccessHandler implementation.
+  bool SupportsRequest(const content::MediaStreamRequest& request,
+                       const extensions::Extension* extension) override;
+  void HandleRequest(content::WebContents* web_contents,
+                     const content::MediaStreamRequest& request,
+                     const content::MediaResponseCallback& callback,
+                     const extensions::Extension* extension) override;
+};
+
+#endif  // CHROME_BROWSER_MEDIA_TAB_CAPTURE_ACCESS_HANDLER_H_