Fix screen capturers to initialize on the same thread on which Start() is called.

webrtc/modules/desktop_capture/desktop_capturer.h

 /*
  *  Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 15 matching lines @@
 class DesktopCapturer {
  public:
   // Interface that must be implemented by the DesktopCapturer consumers.
   class Callback {
    public:
     // Deprecated.
     // TODO(sergeyu): Remove this method once all references to it are removed
     // from chromium.
     virtual SharedMemory* CreateSharedMemory(size_t size) { return nullptr; }
 
+    // Called to notify that the capturer has failed to initialize.
+    virtual void OnInitializationFailed() {};
+
     // Called after a frame has been captured. Handler must take ownership of
     // |frame|. If capture has failed for any reason |frame| is set to NULL
     // (e.g. the window has been closed).
     virtual void OnCaptureCompleted(DesktopFrame* frame) = 0;
 
    protected:
     virtual ~Callback() {}
   };
 
   virtual ~DesktopCapturer() {}
@@ skipping 20 matching lines @@
   // Sets the window to be excluded from the captured image in the future
   // Capture calls. Used to exclude the screenshare notification window for
   // screen capturing.
   virtual void SetExcludedWindow(WindowId window) {}
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_DESKTOP_CAPTURE_DESKTOP_CAPTURER_H_