[BlinkGenPropertyTrees] Only update cc::Layer hierarchy from PAC

This patch stops changing the cc::Layer hierarchy when the
GraphicsLayer hierarchy changes. Instead, we rely on cc::Layers being
updated in PaintArtifactCompositor (PAC).

Bug: 898668
Change-Id: Ib1abb05bc1d8dc7d12ec5f88c8518cf75d077f74
Reviewed-on: https://chromium-review.googlesource.com/c/1313415Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605470}

third_party/blink/renderer/platform/graphics/graphics_layer.cc

@@ -377,7 +377,11 @@ bool GraphicsLayer::PaintWithoutCommit(
 }
 
 void GraphicsLayer::UpdateChildList() {
-  // TODO(pdr): Do not attach cc::Layers when using layer lists.
+  // When using layer lists, cc::Layers are created in PaintArtifactCompositor.
+  if (RuntimeEnabledFeatures::BlinkGenPropertyTreesEnabled() ||
+      RuntimeEnabledFeatures::SlimmingPaintV2Enabled()) {
+    return;
+  }
 
   cc::Layer* child_host = layer_.get();
   child_host->RemoveAllChildren();