Null-check compositing_stacking_context to work around a production crash.

The situation should never happen, but nevertheless seems to.

Bug: 880930

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I1dec6cdb3cb2c004511646a7133c4564a5a4f4ca
Reviewed-on: https://chromium-review.googlesource.com/1211872
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Tien-Ren Chen <trchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589424}

third_party/blink/renderer/core/paint/compositing/composited_layer_mapping.cc

@@ -1482,6 +1482,11 @@ void CompositedLayerMapping::UpdateOverflowControlsHostLayerGeometry(
   LayoutPoint host_layer_position;
 
   if (NeedsToReparentOverflowControls()) {
+    // This should never be true, but for some reason it is.
+    // See https://crbug.com/880930.
+    if (!compositing_stacking_context)
+      return;
+
     CompositedLayerMapping* stacking_clm =
         compositing_stacking_context->GetCompositedLayerMapping();
     DCHECK(stacking_clm);

third_party/blink/renderer/core/paint/compositing/paint_layer_compositor.cc

@@ -398,6 +398,16 @@ GraphicsLayer* PaintLayerCompositor::ParentForContentLayers(
   return GetVisualViewport().ScrollLayer();
 }
 
+#if DCHECK_IS_ON()
+static void AssertWholeTreeNotComposited(const PaintLayer& paint_layer) {
+  DCHECK(paint_layer.GetCompositingState() == kNotComposited);
+  for (PaintLayer* child = paint_layer.FirstChild(); child;
+       child = child->NextSibling()) {
+    AssertWholeTreeNotComposited(*child);
+  }
+}
+#endif
+
 void PaintLayerCompositor::UpdateIfNeeded(
     DocumentLifecycle::LifecycleState target_state,
     CompositingReasonsStats& compositing_reasons_stats) {
@@ -485,6 +495,12 @@ void PaintLayerCompositor::UpdateIfNeeded(
                          ->Parent();
   }
 
+#if DCHECK_IS_ON()
+  if (update_root->GetCompositingState() != kPaintsIntoOwnBacking) {
+    AssertWholeTreeNotComposited(*update_root);
+  }
+#endif
+
   GraphicsLayerUpdater updater;
   updater.Update(*update_root, layers_needing_paint_invalidation);