Change ImmersiveContextMus to use DesktopCaptureClient.

ui/views/widget/desktop_aura/desktop_capture_client.h

Index: ui/views/widget/desktop_aura/desktop_capture_client.h
diff --git a/ui/views/widget/desktop_aura/desktop_capture_client.h b/ui/views/widget/desktop_aura/desktop_capture_client.h
index f2971279d78e495618afb16e961fe98e9539e9e9..f90399295b5141d71568279ddf70ed9a6120e0fa 100644
--- a/ui/views/widget/desktop_aura/desktop_capture_client.h
+++ b/ui/views/widget/desktop_aura/desktop_capture_client.h
@@ -37,6 +37,8 @@ class VIEWS_EXPORT DesktopCaptureClient : public aura::client::CaptureClient {
   explicit DesktopCaptureClient(aura::Window* root);
   ~DesktopCaptureClient() override;
 
+  static aura::Window* GetCaptureWindowGlobal();

[sky, 2017/01/24 23:06:08]

This name is ick! How about instead adding a function get the singleton
DesktopCaptureClient and then call GetGlobalCaptureWindow on it?

[sadrul, 2017/01/25 01:02:13]

I believe the issue is that DesktopCaptureClient is not a singleton; each
DesktopNativeWidgetAura creates its own DesktopCaptureClient [1]. We *could*
have a singleton client, but looks like the current behaviour is deliberate to
work-around some issue on Windows [2].

1:
https://cs.chromium.org/chromium/src/ui/views/widget/desktop_aura/desktop_nat...
2:
https://cs.chromium.org/chromium/src/ui/views/widget/desktop_aura/desktop_cap...

[sky, 2017/01/25 04:47:48]

D'OH! You are certainly right. As I can't think of a better name, please
document that it is exactly the same as GetGlobalCaptureWindow, but static.

+
   // Overridden from aura::client::CaptureClient:
   void SetCapture(aura::Window* window) override;
   void ReleaseCapture(aura::Window* window) override;