[Blink] Invalidate clip path cache if mask layer is mutated

This is a speculative fix for https://crbug.com/856818 .

It is speculated that the clip path of an element become valid without
invalidating its cache, resulting in the associated clip nodes not
being created, and the newly created mask layer having bad layer state.

BUG=856818

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Iab7acc4eb8007d03b259e6dab4f036e2b11bd900
Reviewed-on: https://chromium-review.googlesource.com/1130596Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Tien-Ren Chen <trchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574425}

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

@@ -804,6 +804,12 @@ bool CompositedLayerMapping::UpdateGraphicsLayerConfiguration(
   bool has_clip_path =
       ClipPathClipper::LocalClipPathBoundingBox(GetLayoutObject()).has_value();
   if (UpdateMaskLayer(has_mask || has_clip_path)) {
+    // TODO(crbug.com/856818): Remove this speculative fix.
+    // It is speculated that the clip path of an element become valid without
+    // invalidating its cache, resulting in the associated clip nodes not
+    // being created, and the newly created mask layer having bad layer state.
+    GetLayoutObject().InvalidateClipPathCache();
+
     graphics_layer_->SetMaskLayer(mask_layer_.get());
     layer_config_changed = true;
   }