Speculative fix for crash in document tear-down.

BUG=808341
R=skobes@chromium.org

Change-Id: I298ffd2134ec5021682bc443a5c6a9ba4a9cc13c
Reviewed-on: https://chromium-review.googlesource.com/899928Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534226}

third_party/WebKit/Source/core/page/scrolling/TopDocumentRootScrollerController.cpp

@@ -216,7 +216,7 @@ void TopDocumentRootScrollerController::DidDisposeScrollableArea(
 
   RootFrameViewport* rfv = frame_view->GetRootFrameViewport();
 
-  if (&area == &rfv->LayoutViewport()) {
+  if (rfv && &area == &rfv->LayoutViewport()) {
     DCHECK(frame_view->LayoutViewportScrollableArea());
     rfv->SetLayoutViewport(*frame_view->LayoutViewportScrollableArea());
   }