Check that an instance variable is non-NULL before use.

CID=103659
BUG=NONE
TEST=NONE

Review URL: http://codereview.chromium.org/9836122

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@129478 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/ui/views/tabs/tab_drag_controller2.cc

@@ -981,6 +981,7 @@ void TabDragController2::DetachIntoNewBrowserAndRunMoveLoop(
 
 void TabDragController2::RunMoveLoop() {
   move_loop_browser_view_ = GetAttachedBrowserView();
+  DCHECK(move_loop_browser_view_);
   move_loop_browser_view_->set_move_observer(this);
   is_dragging_window_ = true;
   bool destroyed = false;