Fix segfault when unplugging captured external display

content/browser/media/capture/desktop_capture_device_aura.cc

 // Copyright 2013 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/browser/media/capture/desktop_capture_device_aura.h"
 
 #include "base/logging.h"
 #include "base/metrics/histogram.h"
 #include "base/timer/timer.h"
 #include "cc/output/copy_output_request.h"
@@ skipping 217 matching lines @@
 
   return true;
 }
 
 void DesktopVideoCaptureMachine::Stop(const base::Closure& callback) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
   power_save_blocker_.reset();
 
   // Stop observing compositor and window events.
   if (desktop_window_) {
-    if (desktop_window_->GetHost())
-      desktop_window_->GetHost()->compositor()->RemoveObserver(this);
+    aura::WindowTreeHost* window_host = desktop_window_->GetHost();
+    // In the host destructor the compositor is destroyed before the window.
+    if (window_host && window_host->compositor())
+      window_host->compositor()->RemoveObserver(this);
     desktop_window_->RemoveObserver(this);
     desktop_window_ = NULL;
   }
 
   // Stop timer.
   timer_.Stop();
 
   callback.Run();
 }
 
@@ skipping 282 matching lines @@
     scoped_ptr<Client> client) {
   DVLOG(1) << "Allocating " << params.requested_format.frame_size.ToString();
   core_->AllocateAndStart(params, client.Pass());
 }
 
 void DesktopCaptureDeviceAura::StopAndDeAllocate() {
   core_->StopAndDeAllocate();
 }
 
 }  // namespace content