Bug 1509958, bug 1508734 - Give up on an assert that doesn't hold because of bug 1510208. r=smaug

Can't land the test-case from bug 1510208 as a crashtest because it needs you to
move the mouse around, and as such it doesn't crash in the crashtest harness.

Differential Revision: https://phabricator.services.mozilla.com/D13054

--HG--
extra : moz-landing-system : lando

layout/base/PresShell.cpp

@@ -2101,9 +2101,12 @@ static nsIContent* GetNativeAnonymousSubtreeRoot(nsIContent* aContent)
     return nullptr;
   }
   auto* current = aContent;
+  // FIXME(emilio): This should not need to worry about current being null, but
+  // editor removes nodes in native anonymous subtrees, and we don't clean nodes
+  // from the current event content stack from ContentRemoved, so it can
+  // actually happen, see bug 1510208.
   while (current && !current->IsRootOfNativeAnonymousSubtree()) {
     current = current->GetFlattenedTreeParent();
-    MOZ_DIAGNOSTIC_ASSERT(current, "How?");
   }
   return current;
 }