Fix a crash in ScrollingCoordinator

In a previous CL: https://chromium-review.googlesource.com/c/581907, when
we access to the |composited_layer| inside the for loop, we didn't
perform any null check, and that's causing crash. This CL adds the null
check.

NOTRY=true

Bug: 751389
Change-Id: I5a0861d416b017af5c51ea82ce3add032b0b2316
Reviewed-on: https://chromium-review.googlesource.com/598123
Commit-Queue: Xida Chen <xidachen@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491483}

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

@@ -779,10 +779,12 @@ void ScrollingCoordinator::SetTouchEventTargetRects(
   layers_with_touch_rects_.clear();
   for (const auto& layer_rect : layer_rects) {
     if (!layer_rect.value.IsEmpty()) {
+      DCHECK(layer_rect.key->IsRootLayer() || layer_rect.key->Parent());
       const PaintLayer* composited_layer =
           layer_rect.key
               ->EnclosingLayerForPaintInvalidationCrossingFrameBoundaries();
-      DCHECK(composited_layer);
+      if (!composited_layer)
+        continue;
       layers_with_touch_rects_.insert(composited_layer);
       GraphicsLayer* main_graphics_layer =
           composited_layer->GraphicsLayerBacking(