selectstart event dispatch requires a valid node

Bug: 775320
Test: manual.
Change-Id: I763adcb2ff287cb4594984eec3d494ff93fb8f10
Reviewed-on: https://chromium-review.googlesource.com/723787Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509628}

third_party/WebKit/Source/modules/accessibility/AXLayoutObject.cpp

@@ -2017,9 +2017,10 @@ bool AXLayoutObject::OnNativeSetSelectionAction(const AXRange& selection) {
     return false;
   }
 
-  if (anchor_object->GetLayoutObject()->GetNode()->DispatchEvent(
+  if (anchor_object->GetLayoutObject()->GetNode() &&
+      anchor_object->GetLayoutObject()->GetNode()->DispatchEvent(
           Event::CreateCancelableBubble(EventTypeNames::selectstart)) !=
-      DispatchEventResult::kNotCanceled)
+          DispatchEventResult::kNotCanceled)
     return false;
 
   if (!IsValidSelectionBound(anchor_object) ||