make StackingClient dispatch to either ash or desktop

chrome/browser/ui/browser_tabstrip.cc

Index: chrome/browser/ui/browser_tabstrip.cc
diff --git a/chrome/browser/ui/browser_tabstrip.cc b/chrome/browser/ui/browser_tabstrip.cc
index d060db9f9d88bb6acc6a5b133bbbeb90619f6c67..9273b7cc1e957b1c69dcc90d6e2ee16cd7cac5af 100644
--- a/chrome/browser/ui/browser_tabstrip.cc
+++ b/chrome/browser/ui/browser_tabstrip.cc
@@ -56,6 +56,11 @@ void ActivateTabAt(Browser* browser, int index, bool user_gesture) {
 }
 
 void AddBlankTabAt(Browser* browser, int index, bool foreground) {
+  // TODO(scottmg): http://crbug.com/128578
+  // This is necessary because WebContentsViewAura doesn't have enough context
+  // to get the right StackingClient (and therefore parent window) otherwise.
+  ScopedForceDesktopType force_desktop_type(browser->host_desktop_type());
+
   // Time new tab page creation time.  We keep track of the timing data in
   // WebContents, but we want to include the time it takes to create the
   // WebContents object too.