Speculative fix for crash in blink::DragData::modifiers.

BUG=671152

Review-Url: https://codereview.chromium.org/2844243003
Cr-Commit-Position: refs/heads/master@{#468184}

third_party/WebKit/Source/web/WebFrameWidgetBase.cpp

@@ -80,7 +80,12 @@ void WebFrameWidgetBase::DragTargetDragLeave(const WebPoint& point_in_viewport,
                                              const WebPoint& screen_point) {
   DCHECK(current_drag_data_);
 
-  if (IgnoreInputEvents()) {
+  // TODO(paulmeyer): It shouldn't be possible for |m_currentDragData| to be
+  // null here, but this is somehow happening (rarely). This suggests that in
+  // some cases drag-leave is happening before drag-enter, which should be
+  // impossible. This needs to be investigated further. Once fixed, the extra
+  // check for |!m_currentDragData| should be removed. (crbug.com/671152)
+  if (IgnoreInputEvents() || !current_drag_data_) {
     CancelDrag();
     return;
   }