Fix another d&d crash that happens when the tab contents

webkit/glue/webview_impl.cc

Index: webkit/glue/webview_impl.cc
diff --git a/webkit/glue/webview_impl.cc b/webkit/glue/webview_impl.cc
index 53a5f228d11b790e22e2c5b2dedba836bfd9074c..0556ac8e80b79b462ccc9c435630195bff37bf24 100644
--- a/webkit/glue/webview_impl.cc
+++ b/webkit/glue/webview_impl.cc
@@ -1588,9 +1588,12 @@ void WebViewImpl::DragSourceMovedTo(
 }
 
 void WebViewImpl::DragSourceSystemDragEnded() {
-  page_->dragController()->dragEnded();
-  DCHECK(doing_drag_and_drop_);
-  doing_drag_and_drop_ = false;
+  // It's possible for us to get this callback while not doing a drag if
+  // it's from a previous page that got unloaded.
+  if (doing_drag_and_drop_) {
+    page_->dragController()->dragEnded();
+    doing_drag_and_drop_ = false;
+  }
 }
 
 bool WebViewImpl::DragTargetDragEnter(