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

webrtc/modules/desktop_capture/desktop_capturer.h

Index: webrtc/modules/desktop_capture/desktop_capturer.h
diff --git a/webrtc/modules/desktop_capture/desktop_capturer.h b/webrtc/modules/desktop_capture/desktop_capturer.h
index 47f78dc3ff411510aabc417045419e010605487b..c6c3c7bea0ab8ac7268c1c840c23e324600af2fb 100644
--- a/webrtc/modules/desktop_capture/desktop_capturer.h
+++ b/webrtc/modules/desktop_capture/desktop_capturer.h
@@ -33,6 +33,9 @@ class DesktopCapturer {
     // 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).