I can probably have a deeper look at this tomorrow, but I have confirmed that a DCHECK crash I am seeing on trunk is caused by r535797. Reverting it fixes the problem.

I can reproduce consistently by just visiting https://www.youtube.com/unboxed using a build with DCHECKs enabled and with the --site-per-process flag. It crashes within 10-20 seconds with the following stack:
[1:1:0214/133750.045749:FATAL:PaintLayer.cpp(2563)] Check failed: IsAllowedToQueryCompositingState(). 
#0 0x7f7f698389fd base::debug::StackTrace::StackTrace()
#1 0x7f7f69836eec base::debug::StackTrace::StackTrace()
#2 0x7f7f698bf21a logging::LogMessage::~LogMessage()
#3 0x7f7f574111aa blink::PaintLayer::GetCompositingState()
#4 0x7f7f5741bca0 blink::PaintLayer::GraphicsLayerBacking()
#5 0x7f7f573934fe blink::ScrollingCoordinator::SetTouchEventTargetRects()
#6 0x7f7f5738f203 blink::ScrollingCoordinator::UpdateTouchEventTargetRectsIfNeeded()
#7 0x7f7f5738e439 blink::ScrollingCoordinator::UpdateAfterCompositingChangeIfNeeded()
#8 0x7f7f56ab63c5 blink::LocalFrameView::UpdateLifecyclePhasesInternal()
#9 0x7f7f56ab5a52 blink::LocalFrameView::UpdateAllLifecyclePhases()
#10 0x7f7f57366c4b blink::PageAnimator::UpdateAllLifecyclePhases()
#11 0x7f7f5736c2ec blink::PageWidgetDelegate::UpdateLifecycle()
#12 0x7f7f56b2e10e blink::WebFrameWidgetImpl::UpdateLifecycle()
#13 0x7f7f6503ed06 content::RenderWidget::UpdateVisualState()
#14 0x7f7f64e2bee0 content::RenderWidgetCompositor::UpdateLayerTreeHost()
#15 0x7f7f611161e0 cc::LayerTreeHost::RequestMainFrameUpdate()
#16 0x7f7f61206200 cc::ProxyMain::BeginMainFrame()

It is strange that it only repros with --site-per-process, but it is the main frame that is crashing, as if the presence of the remote frame is somehow causing the problem.