Support dragging panels to stack and snap.

chrome/browser/ui/panels/panel_collection.h

Index: chrome/browser/ui/panels/panel_collection.h
diff --git a/chrome/browser/ui/panels/panel_collection.h b/chrome/browser/ui/panels/panel_collection.h
index 70d3639f943faea8fc853f8a6c074953b800b86c..280d5aba64f44ab65c6eadc0dc5f13f0bdf3cf20 100644
--- a/chrome/browser/ui/panels/panel_collection.h
+++ b/chrome/browser/ui/panels/panel_collection.h
@@ -29,12 +29,17 @@ class PanelCollection {
     DEFAULT_POSITION = 0x0,
     // The panel is being added based on its current known position.
     KNOWN_POSITION = 0x1,
+    // The panel is added and placed at top position (currently only used by
+    // stacked collection)
+    TOP_POSITION = 0x2,
     // Do not update panel bounds. Only valid with DEFAULT_POSIITON.
-    DO_NOT_UPDATE_BOUNDS = 0x2,
+    DO_NOT_UPDATE_BOUNDS = 0x4,
     // Wait for a brief delay before refreshing layout of the collection after
     // adding panel to the collection. If not set, the collection will refresh
     // its layout immediately.
-    DELAY_LAYOUT_REFRESH = 0x4,
+    DELAY_LAYOUT_REFRESH = 0x8,
+    // Do not refresh layout. Used by stacking.
+    NO_LAYOUT_REFRESH = 0x10
   };
 
   Type type() const { return type_; }