Make showLayerTree(PaintLayer) work in CompositeAfterPaint

Previously DisableCompositingQueryAsserts triggered DCHECK failure in
CompositeAfterPaint.

Change-Id: I9f341a7fb176d357aa325f2cde9c497cf4536fbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2261176
Auto-Submit: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781490}

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

@@ -3514,12 +3514,15 @@ DisableCompositingQueryAsserts::DisableCompositingQueryAsserts()
 
 #if DCHECK_IS_ON()
 void showLayerTree(const blink::PaintLayer* layer) {
-  blink::DisableCompositingQueryAsserts disabler;
   if (!layer) {
     LOG(ERROR) << "Cannot showLayerTree. Root is (nil)";
     return;
   }
 
+  base::Optional<blink::DisableCompositingQueryAsserts> disabler;
+  if (!blink::RuntimeEnabledFeatures::CompositeAfterPaintEnabled())
+    disabler.emplace();
+
   if (blink::LocalFrame* frame = layer->GetLayoutObject().GetFrame()) {
     WTF::String output =
         ExternalRepresentation(frame,