All the following layout tests are crashing with --root-layer-scrolls:

http/tests/devtools/layers/layer-canvas-log.html
http/tests/devtools/layers/layer-compositing-reasons.html
http/tests/devtools/layers/layer-replay-scale.html
http/tests/devtools/layers/layer-scroll-rects-get.html
http/tests/devtools/layers/layer-sticky-position-constraint-get.html
http/tests/devtools/layers/layer-tree-model.html
http/tests/devtools/layers/layers-3d-view-hit-testing.html
http/tests/devtools/layers/layers-panel-mouse-events.html
http/tests/devtools/layers/no-overlay-layers.html

These constitute the majority of crashing layout tests for RLS.  They're probably all crashing for the same reason.  Here's an example crash stack:

#4 0x7fbb8344034c blink::GraphicsLayer::PlatformLayer()
#5 0x7fbb85931188 blink::InspectorLayerTreeAgent::BuildLayerTree()
#6 0x7fbb85930ea3 blink::InspectorLayerTreeAgent::LayerTreeDidChange()
#7 0x7fbb85930e5c blink::InspectorLayerTreeAgent::enable()
#8 0x7fbb85930fb0 blink::InspectorLayerTreeAgent::enable()
#9 0x7fbb86775c5c blink::protocol::LayerTree::DispatcherImpl::enable()
#10 0x7fbb86775abd blink::protocol::LayerTree::DispatcherImpl::dispatch()
#11 0x7fbb8681edad blink::protocol::UberDispatcher::dispatch()
#12 0x7fbb8598e2b0 blink::InspectorSession::DispatchProtocolMessage()
#13 0x7fbb8533d4ee blink::WebDevToolsAgentImpl::DispatchMessageFromFrontend()
#14 0x7fbb8533d34e blink::WebDevToolsAgentImpl::DispatchOnInspectorBackend()
#15 0x7fbb906e5466 content::DevToolsAgent::OnDispatchOnInspectorBackend()
...

We should figure out what's going on here.