mus Window Server: implement event capture

components/mus/ws/event_dispatcher_delegate.h

Index: components/mus/ws/event_dispatcher_delegate.h
diff --git a/components/mus/ws/event_dispatcher_delegate.h b/components/mus/ws/event_dispatcher_delegate.h
index 5ba2152a43891949ca58c8475f5820ae778fbe54..469e72af90b0f0a5926944e04b880f9cdaa94d52 100644
--- a/components/mus/ws/event_dispatcher_delegate.h
+++ b/components/mus/ws/event_dispatcher_delegate.h
@@ -22,6 +22,10 @@ class EventDispatcherDelegate {
   virtual void SetFocusedWindowFromEventDispatcher(ServerWindow* window) = 0;
   virtual ServerWindow* GetFocusedWindowForEventDispatcher() = 0;
 
+  virtual void SetCapture(ServerWindow* window) = 0;

[sky, 2016/02/10 17:38:22]

Clarify when these are called. In particular it's not clear when ReleaseCapture
vs OnLostCapture is called.
Also, I wouldn't supply a window to SetCapture/ReleaseCapture unless you need
it. It helps clarify this is intended to set/release capture on the native
display. In fact maybe the two should be called
SetNativeCapture/ReleaseNativeCapture.

[jonross, 2016/02/10 19:45:49]

Done.

As discussed offline the two OnLostCapture methods have been renamed to make
them more clear as to their functionality

+  virtual void ReleaseCapture(ServerWindow* window) = 0;
+  virtual void OnLostCapture(ServerWindow* window) = 0;
+
   // |in_nonclient_area| is true if the event occurred in the non-client area.
   virtual void DispatchInputEventToWindow(ServerWindow* target,
                                           bool in_nonclient_area,