MacViews: Fix failing mouse capture unittests.

ui/views/cocoa/cocoa_mouse_capture.h

Index: ui/views/cocoa/cocoa_mouse_capture.h
diff --git a/ui/views/cocoa/cocoa_mouse_capture.h b/ui/views/cocoa/cocoa_mouse_capture.h
index 247caa4e82caa1200f224959b48fdb826bf5e9db..9b170e4a7e9c41b1b48c91dc31868cdb26bdb54c 100644
--- a/ui/views/cocoa/cocoa_mouse_capture.h
+++ b/ui/views/cocoa/cocoa_mouse_capture.h
@@ -10,6 +10,8 @@
 #include "base/macros.h"
 #include "ui/views/views_export.h"
 
+@class NSWindow;
+
 namespace views {
 
 class CocoaMouseCaptureDelegate;
@@ -24,6 +26,10 @@ class VIEWS_EXPORT CocoaMouseCapture {
   explicit CocoaMouseCapture(CocoaMouseCaptureDelegate* delegate);
   ~CocoaMouseCapture();
 
+  // Returns the NSWindow with capture or nil if no window has capture
+  // currently.
+  static NSWindow* GetGlobalCaptureWindow();
+
   // True if the event tap is active (i.e. not stolen by a later instance).
   bool IsActive() const { return !!active_handle_; }