Bug 1471671 - Avoid crash with WebRender when the scroll metadata is unavailable. r=kats

We are seeing crash reports in the wild where there is no scroll
metadata available for an ASR for a display item and its clip. It
appears that in the non-WR path, it skips such items, so we should
probably do the same thing with WebRender. If the scrolling ends up
being wrong, hopefully a reproducible use case will make its way to use
to further debug, as the crash reports have not yielded anything to date.

Differential Revision: https://phabricator.services.mozilla.com/D9699

gfx/layers/wr/ClipManager.cpp

@@ -292,9 +292,12 @@ ClipManager::DefineScrollLayers(const ActiveScrolledRoot* aASR,
 
   Maybe<ScrollMetadata> metadata = aASR->mScrollableFrame->ComputeScrollMetadata(
       mManager, aItem->ReferenceFrame(), Nothing(), nullptr);
-  MOZ_ASSERT(metadata);
-  FrameMetrics& metrics = metadata->GetMetrics();
+  if (!metadata) {
+    MOZ_ASSERT_UNREACHABLE("Expected scroll metadata to be available!");
+    return ancestorScrollId;
+  }
 
+  FrameMetrics& metrics = metadata->GetMetrics();
   if (!metrics.IsScrollable()) {
     // This item is a scrolling no-op, skip over it in the ASR chain.
     return ancestorScrollId;