[Blink] Fix dangling pointer to destroyed child clipping mask layer

The contents clipping mask layer on the main graphics layer needs to be
updated unconditionally because otherwise we can have dangling pointer
to a destroyed mask when both the mask and the contents layer are
destroyed at the same time.

BUG=807240

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Ica51387507232c20f1deacf9572546490f587eb9
Reviewed-on: https://chromium-review.googlesource.com/894194
Commit-Queue: Tien-Ren Chen <trchen@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533080}

third_party/WebKit/Source/core/paint/compositing/CompositedLayerMapping.cpp

@@ -806,10 +806,7 @@ bool CompositedLayerMapping::UpdateGraphicsLayerConfiguration(
       ScrollingLayer()->SetMaskLayer(first_come_first_served);
       first_come_first_served = nullptr;
     }
-    if (is_accelerated_contents) {
-      graphics_layer_->SetContentsClippingMaskLayer(first_come_first_served);
-      first_come_first_served = nullptr;
-    }
+    graphics_layer_->SetContentsClippingMaskLayer(first_come_first_served);
   }
 
   UpdateBackgroundColor();