Enables using the magnification API for screen capturing on Windows under a Finch experiment.

content/browser/media/capture/desktop_capture_device.h

Index: content/browser/media/capture/desktop_capture_device.h
diff --git a/content/browser/media/capture/desktop_capture_device.h b/content/browser/media/capture/desktop_capture_device.h
index af16bda4127821922924ce8868d23e6dbf4a98be..9187517086f96634429678b59490c450a62a9286 100644
--- a/content/browser/media/capture/desktop_capture_device.h
+++ b/content/browser/media/capture/desktop_capture_device.h
@@ -14,6 +14,7 @@
 
 namespace base {
 class SequencedTaskRunner;
+class Thread;
 }  // namespace base
 
 namespace webrtc {
@@ -33,9 +34,15 @@ class CONTENT_EXPORT DesktopCaptureDevice : public media::VideoCaptureDevice {
   static scoped_ptr<media::VideoCaptureDevice> Create(
       const DesktopMediaID& source);
 
+  // Run the capturer on the |task_runner|.
   DesktopCaptureDevice(scoped_refptr<base::SequencedTaskRunner> task_runner,
                        scoped_ptr<webrtc::DesktopCapturer> desktop_capturer,
                        DesktopMediaID::Type type);
+  // Run the capturer on a dedicated thread. Needed for using the magnification
+  // API on Windows.
+  DesktopCaptureDevice(scoped_ptr<base::Thread> thread,

[Sergey Ulanov, 2014/05/05 20:10:06]

It's better to avoid overriding constructor. I suggest making constructor
private and always passing both |task_runner| and |thread|. The constructor is
called from Create() and from unittests. So you'll need to befriend this class
with DesktopCaptureDeviceTest.

+                       scoped_ptr<webrtc::DesktopCapturer> desktop_capturer,
+                       DesktopMediaID::Type type);
   virtual ~DesktopCaptureDevice();
 
   // VideoCaptureDevice interface.