Don't unregister non-elements.

Some layers under --enable-bloink-gen-property-trees at least
have no element_id.  Don't unregister them since they were never
registered.

Bug: None
Cq-Include-Trybots: luci.chromium.try:linux-blink-gen-property-trees;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I1d9cf412e56ce088819157f7b43d5214fdcf39d7
Reviewed-on: https://chromium-review.googlesource.com/1155821Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Peter Mayo <petermayo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579254}

third_party/blink/renderer/platform/graphics/compositing/paint_artifact_compositor.cc

@@ -84,7 +84,9 @@ void PaintArtifactCompositor::RemoveChildLayers() {
   if (!host)
     return;
   for (auto child : root_layer_->children()) {
-    host->UnregisterElement(child->element_id(), cc::ElementListType::ACTIVE);
+    auto element_id = child->element_id();
+    if (element_id)
+      host->UnregisterElement(element_id, cc::ElementListType::ACTIVE);
   }
   root_layer_->RemoveAllChildren();
   if (extra_data_for_testing_enabled_) {