Switch Extensions::TabHelper to use WebContents, WebContentsUserData.

chrome/browser/ui/gtk/tabs/dragged_view_gtk.cc

Index: chrome/browser/ui/gtk/tabs/dragged_view_gtk.cc
diff --git a/chrome/browser/ui/gtk/tabs/dragged_view_gtk.cc b/chrome/browser/ui/gtk/tabs/dragged_view_gtk.cc
index dc6986950db41532d1a24148a229e9f5f987e8d4..5a31b22443a930b5ad290b5ea20aa63b46afe7c7 100644
--- a/chrome/browser/ui/gtk/tabs/dragged_view_gtk.cc
+++ b/chrome/browser/ui/gtk/tabs/dragged_view_gtk.cc
@@ -71,11 +71,10 @@ DraggedViewGtk::DraggedViewGtk(DragData* drag_data,
   }
 
   for (size_t i = 0; i < drag_data_->size(); i++) {
-    TabContents* tab_contents = TabContents::FromWebContents(
-        drag_data_->get(i)->contents_->web_contents());
+    WebContents* web_contents = drag_data_->get(i)->contents_->web_contents();
     renderers_[i]->UpdateData(
-        drag_data_->get(i)->contents_->web_contents(),
-        tab_contents->extension_tab_helper()->is_app(),
+        web_contents,
+        extensions::TabHelper::FromWebContents(web_contents)->is_app(),
         false); // loading_only
     renderers_[i]->set_is_active(
         static_cast<int>(i) == drag_data_->source_tab_index());