mus: Use the new drag API to implement tab dragging in chrome.

chrome/browser/ui/views/tabs/window_finder.h

Index: chrome/browser/ui/views/tabs/window_finder.h
diff --git a/chrome/browser/ui/views/tabs/window_finder.h b/chrome/browser/ui/views/tabs/window_finder.h
index 160fb6a3c5ffca56728778f226e426baa4e236c7..4ce91d50ed01aad9fa9b789755e9e2c4bbda835e 100644
--- a/chrome/browser/ui/views/tabs/window_finder.h
+++ b/chrome/browser/ui/views/tabs/window_finder.h
@@ -17,11 +17,17 @@ namespace gfx {
 class Point;
 }
 
+// Class used by the tabstrip to find chrome windows that we can attach tabs
+// to.
 class WindowFinder {
  public:
   WindowFinder();
   virtual ~WindowFinder();
 
+  // Finds the topmost visible chrome window at |screen_point|. This should
+  // return nullptr if |screen_point| is in another program's window which
+  // occludes the topmost chrome window. Ignores the windows in |ignore|, which
+  // contain windows such as the tab being dragged right now.
   virtual gfx::NativeWindow GetLocalProcessWindowAtPoint(
       const gfx::Point& screen_point,
       const std::set<gfx::NativeWindow>& ignore);