This bug is for crash report bp-b5982a0b-ad84-40da-b87c-4a0000190323.

Top 10 frames of crashing thread:

0 xul.dll mozilla::layers::WebRenderScrollData::GetLayerCount 
1 xul.dll class mozilla::layers::WebRenderScrollDataWrapper mozilla::layers::WebRenderScrollDataWrapper::GetLastChild gfx/layers/wr/WebRenderScrollDataWrapper.h:177
2 xul.dll static void mozilla::layers::ForEachNode<mozilla::layers::ReverseIterator, mozilla::layers::WebRenderScrollDataWrapper, `lambda at z:/task_1553335548/build/src/gfx/layers/apz/src/APZCTreeManager.cpp:413:9', `lambda at z:/task_1553335548/build/src/gfx/layers/apz/src/APZCTreeManager.cpp:487:9'> gfx/layers/TreeTraversal.h:140
3 xul.dll static void mozilla::layers::ForEachNode<mozilla::layers::ReverseIterator, mozilla::layers::WebRenderScrollDataWrapper, `lambda at z:/task_1553335548/build/src/gfx/layers/apz/src/APZCTreeManager.cpp:413:9', `lambda at z:/task_1553335548/build/src/gfx/layers/apz/src/APZCTreeManager.cpp:487:9'> gfx/layers/TreeTraversal.h:142
4 xul.dll static void mozilla::layers::APZCTreeManager::UpdateHitTestingTreeImpl<mozilla::layers::WebRenderScrollDataWrapper> gfx/layers/apz/src/APZCTreeManager.cpp:411
5 xul.dll static void mozilla::layers::APZUpdater::UpdateScrollDataAndTreeState::<unnamed-tag>::operator gfx/layers/apz/src/APZUpdater.cpp:209
6 xul.dll nsresult mozilla::detail::RunnableFunction<`lambda at z:/task_1553335548/build/src/gfx/layers/apz/src/APZUpdater.cpp:203:11'>::Run xpcom/threads/nsThreadUtils.h:562
7 xul.dll void mozilla::layers::APZUpdater::ProcessQueue gfx/layers/apz/src/APZUpdater.cpp:524
8 xul.dll static void mozilla::layers::APZUpdater::CompleteSceneSwap gfx/layers/apz/src/APZUpdater.cpp:121
9 xul.dll apz_post_scene_swap gfx/layers/apz/src/APZUpdater.cpp:571

There are 4 crashes (from 2 installations) in nightly 68 with buildid 20190323094805. In analyzing the backtrace, the regression may have been introduced by patch [1] to fix bug 1441308.

[1] https://hg.mozilla.org/mozilla-central/rev?node=96da9d241051

We're missing a null check. I can fix. This should only be getting hit if the user has turned on document splitting, which is off by default.

This has no functional effect but makes it consistent with other similar
sites.

Depends on D24650

https://hg.mozilla.org/mozilla-central/rev/44f442f26b6d
https://hg.mozilla.org/mozilla-central/rev/f33d20b3893f