[Blink] Workaround nullptr properties on mask layers

This CL workaround https://crbug.com/856818 . We don't know how we ended
up having a mask layer without an applicable mask or clip-path being
found during property tree building, but at least we should not crash.

BUG=856818

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ib62e0ebd6fad4f52a05a57bb4277b4411f3fecfd
Reviewed-on: https://chromium-review.googlesource.com/1178988Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Tien-Ren Chen <trchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583903}

third_party/blink/renderer/core/paint/paint_property_tree_builder.cc

@@ -612,14 +612,26 @@ void FragmentPaintPropertyTreeBuilder::UpdateEffect() {
           object_, context_.current.paint_offset);
       bool has_clip_path =
           style.ClipPath() && fragment_data_.ClipPathBoundingBox();
-      bool has_spv1_composited_clip_path =
-          has_clip_path && object_.HasLayer() &&
+      bool is_spv1_composited =
+          object_.HasLayer() &&
           ToLayoutBoxModelObject(object_).Layer()->GetCompositedLayerMapping();
+      bool has_spv1_composited_clip_path = has_clip_path && is_spv1_composited;
       bool has_mask_based_clip_path =
           has_clip_path && !fragment_data_.ClipPathPath();
       base::Optional<IntRect> clip_path_clip;
       if (has_spv1_composited_clip_path || has_mask_based_clip_path) {
         clip_path_clip = fragment_data_.ClipPathBoundingBox();
+      } else if (!mask_clip && is_spv1_composited &&
+                 ToLayoutBoxModelObject(object_)
+                     .Layer()
+                     ->GetCompositedLayerMapping()
+                     ->MaskLayer()) {
+        // TODO(crbug.com/856818): This should never happen.
+        // This is a band-aid to avoid nullptr properties on the mask layer
+        // crashing the renderer, but will result in incorrect rendering.
+        NOTREACHED();
+        has_spv1_composited_clip_path = true;
+        clip_path_clip = IntRect();
       }
       if (mask_clip || clip_path_clip) {
         IntRect combined_clip = mask_clip ? *mask_clip : *clip_path_clip;