Bug 1599606 - Remove an assert in non-WebRender code. r=jnicol

This assert fails on the new reftest from this bug, on Android.

I haven't dug into it too deeply, given that it's code that'll be going away at
some point soon (hopefully), but I think what happens is that we have a fixed
layer which is not annotated as fixed. That's normal for background-attachment:
fixed root backgrounds. We handle explicitly-annotated-as-fixed layers a bit
further up in this function.

Returning false here seems like a very safe thing to do.

Depends on D54855

Differential Revision: https://phabricator.services.mozilla.com/D103736

gfx/layers/client/ClientTiledPaintedLayer.cpp

@@ -284,8 +284,6 @@ bool ClientTiledPaintedLayer::UseProgressiveDraw() {
   if (ClientManager()->AsyncPanZoomEnabled()) {
     LayerMetricsWrapper scrollAncestor;
     GetAncestorLayers(&scrollAncestor, nullptr, nullptr);
-    MOZ_ASSERT(
-        scrollAncestor);  // because mPaintData.mCriticalDisplayPort is set
     if (!scrollAncestor) {
       return false;
     }