Fix "unused variable" warnings.

content/browser/media/capture/window_activity_tracker_mac.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 CONTENT_BROWSER_MEDIA_CAPTURE_WINDOW_ACTIVITY_TRACKER_MAC_H_
 #define CONTENT_BROWSER_MEDIA_CAPTURE_WINDOW_ACTIVITY_TRACKER_MAC_H_
 
 #import <AppKit/AppKit.h>
 #import <CoreFoundation/CoreFoundation.h>
 
@@ skipping 31 matching lines @@
 class CONTENT_EXPORT WindowActivityTrackerMac : public WindowActivityTracker {
  public:
   explicit WindowActivityTrackerMac(NSView* view);
   ~WindowActivityTrackerMac() final;
 
   base::WeakPtr<WindowActivityTracker> GetWeakPtr() final;
 
  private:
   void OnMouseActivity();
 
-  NSView* const view_;
   base::scoped_nsobject<MouseTracker> mouse_tracker_;
 
   base::WeakPtrFactory<WindowActivityTrackerMac> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowActivityTrackerMac);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_MEDIA_CAPTURE_WINDOW_ACTIVITY_TRACKER_MAC_H_