[BlinkGenPropertyTrees] DCHECK that use_layer_lists is enabled

In [1] we found tests were being run with BlinkGenPropertyTrees enabled
but without use_layer_lists. This issue is difficult to debug and will
hit other developers. This patch adds a DCHECK that use_layer_lists is
enabled.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1083012

Bug: 836902
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I0c9cc1393100a66528abae84842e2164fb4ad107
Reviewed-on: https://chromium-review.googlesource.com/1102562Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568012}

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

@@ -702,6 +702,12 @@ void PaintArtifactCompositor::Update(
   if (!host)
     return;
 
+  // When using BlinkGenPropertyTrees, the compositor accepts a list of layers
+  // and property trees instead of building property trees. This DCHECK ensures
+  // we have not forgotten to set |use_layer_lists|.
+  DCHECK(!RuntimeEnabledFeatures::BlinkGenPropertyTreesEnabled() ||
+         host->GetSettings().use_layer_lists);
+
   if (extra_data_for_testing_enabled_)
     extra_data_for_testing_.reset(new ExtraDataForTesting);