Fixes possible crash in tab dragging

This crash would happen if you select a couple of tabs, then click but
don't drag and hit escape or control-w. The crash would occur because
we would end up inserting the tabs a second time even though they are
already in the tabstrip.

BUG=360822
TEST=see bug
R=ben@chromium.org

Review URL: https://codereview.chromium.org/548703002

Cr-Commit-Position: refs/heads/master@{#293625}

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

@@ -1358,7 +1358,8 @@ void TabDragController::EndDragImpl(EndDragType type) {
     }
   } else if (drag_data_.size() > 1) {
     initial_selection_model_.Clear();
-    RevertDrag();
+    if (started_drag_)
+      RevertDrag();
   }  // else case the only tab we were dragging was deleted. Nothing to do.
 
   // Clear out drag data so we don't attempt to do anything with it.