Several layout tests appear to fail with an asserting DCHECK, appears this is related to the fact we haven't hooked up the visual viewport/page scale layers.

e.g. ./third_party/WebKit/Tools/Scripts/run-webkit-tests -t ChromeDebug --additional-driver-flag="--enable-blink-gen-property-trees" scrollbars/scrollbar-removed-by-viewport-crash.html

crash log for renderer (pid <unknown>):
STDOUT: <empty>
STDERR: [1:1:0509/112655.798827:FATAL:layer_tree_impl.cc(849)] Check failed: layer_list_.empty() || *page_scale_factor == 1. 
STDERR: #0 0x7f774f6329bd base::debug::StackTrace::StackTrace()
STDERR: #1 0x7f774f35c4bc base::debug::StackTrace::StackTrace()
STDERR: #2 0x7f774f3cdfaa logging::LogMessage::~LogMessage()
STDERR: #3 0x7f7749765797 cc::LayerTreeImpl::PushPageScaleFactorAndLimits()
STDERR: #4 0x7f7749769222 cc::LayerTreeImpl::PushPageScaleFromMainThread()
STDERR: #5 0x7f77496f24da cc::LayerTreeHost::PushLayerTreePropertiesTo()
STDERR: #6 0x7f77496f1596 cc::LayerTreeHost::FinishCommitOnImplThread()
STDERR: #7 0x7f77497f34ee cc::SingleThreadProxy::DoCommit()
STDERR: #8 0x7f77497f6e4a cc::SingleThreadProxy::CompositeImmediately()
STDERR: #9 0x7f77496f60e5 cc::LayerTreeHost::Composite()
STDERR: #10 0x7f774d685136 content::RenderWidgetCompositor::SynchronouslyComposite()
STDERR: #11 0x7f774d686167 content::RenderWidgetCompositor::SynchronouslyCompositeNoRasterForTesting()
STDERR: #12 0x7f773f8f3779 blink::WebViewImpl::UpdateAllLifecyclePhasesAndCompositeForTesting()
STDERR: #13 0x7f773a502f0d test_runner::WebWidgetTestClient::AnimateNow()
STDERR: #14 0x7f773a47eabf _ZN4base8internal13FunctorTraitsIMN11test_runner23MockWebSpeechRecognizerEFvvEvE6InvokeIS5_RKNS_7WeakPtrIS3_EEJEEEvT_OT0_DpOT1_
STDERR: #15 0x7f773a47ea3a _ZN4base8internal12InvokeHelperILb1EvE8MakeItSoIRKMN11test_runner23MockWebSpeechRecognizerEFvvERKNS_7WeakPtrIS5_EEJEEEvOT_OT0_DpOT1_
STDERR: #16 0x7f773a47e9d0 _ZN4base8internal7InvokerINS0_9BindStateIMN11test_runner23MockWebSpeechRecognizerEFvvEJNS_7WeakPtrIS4_EEEEEFvvEE7RunImplIRKS6_RKNSt3__15tupleIJS8_EEEJLm0EEEEvOT_OT0_NSF_16integer_sequenceImJXspT1_EEEE
STDERR: #17 0x7f773a47e91c _ZN4base8internal7InvokerINS0_9BindStateIMN11test_runner23MockWebSpeechRecognizerEFvvEJNS_7WeakPtrIS4_EEEEEFvvEE3RunEPNS0_13BindStateBaseE
STDERR: #18 0x7f774f30b1ee _ZNO4base12OnceCallbackIFvvEE3RunEv
STDERR: #19 0x7f774f35d982 base::debug::TaskAnnotator::RunTask()
STDERR: #20 0x7f773d40052d blink::scheduler::internal::ThreadControllerImpl::DoWork()
STDERR: #21 0x7f773d4032f1 _ZN4base8internal13FunctorTraitsIMN5blink9scheduler8internal20ThreadControllerImplEFvNS4_19SequencedTaskSource8WorkTypeEEvE6InvokeIS9_RKNS_7WeakPtrIS5_EEJRKS7_EEEvT_OT0_DpOT1_
STDERR: #22 0x7f773d403255 _ZN4base8internal12InvokeHelperILb1EvE8MakeItSoIRKMN5blink9scheduler8internal20ThreadControllerImplEFvNS6_19SequencedTaskSource8WorkTypeEERKNS_7WeakPtrIS7_EEJRKS9_EEEvOT_OT0_DpOT1_
STDERR: #23 0x7f773d4031cd _ZN4base8internal7InvokerINS0_9BindStateIMN5blink9scheduler8internal20ThreadControllerImplEFvNS5_19SequencedTaskSource8WorkTypeEEJNS_7WeakPtrIS6_EES8_EEEFvvEE7RunImplIRKSA_RKNSt3__15tupleIJSC_S8_EEEJLm0ELm1EEEEvOT_OT0_NSJ_16integer_sequenceImJXspT1_EEEE
STDERR: #24 0x7f773d4030dc _ZN4base8internal7InvokerINS0_9BindStateIMN5blink9scheduler8internal20ThreadControllerImplEFvNS5_19SequencedTaskSource8WorkTypeEEJNS_7WeakPtrIS6_EES8_EEEFvvEE3RunEPNS0_13BindStateBaseE
STDERR: #25 0x7f774f30b1ee _ZNO4base12OnceCallbackIFvvEE3RunEv
STDERR: #26 0x7f774f35d982 base::debug::TaskAnnotator::RunTask()
STDERR: #27 0x7f774f3ed5a9 base::internal::IncomingTaskQueue::RunTask()
STDERR: #28 0x7f774f3f6687 base::MessageLoop::RunTask()
STDERR: #29 0x7f774f3f68f8 base::MessageLoop::DeferOrRunPendingTask()
STDERR: #30 0x7f774f3f6c29 base::MessageLoop::DoWork()
STDERR: #31 0x7f774f3fa257 base::MessagePumpDefault::Run()
STDERR: #32 0x7f774f3f5e7b base::MessageLoop::Run()
STDERR: #33 0x7f774f49f64d base::RunLoop::Run()
STDERR: #34 0x7f774daa1a65 content::RendererMain()
STDERR: #35 0x7f774dcdb2c6 content::RunZygote()
STDERR: #36 0x7f774dcdde9e content::RunNamedProcessTypeMain()
STDERR: #37 0x7f774dce1834 content::ContentMainRunnerImpl::Run()
STDERR: #38 0x7f774dcd49f5 content::ContentServiceManagerMainDelegate::RunEmbedderProcess()
STDERR: #39 0x7f7743583dbc service_manager::Main()
STDERR: #40 0x7f774dcdac05 content::ContentMain()
STDERR: #41 0x000000658153 main
STDERR: #42 0x7f773238c2b1 __libc_start_main
STDERR: #43 0x00000065802a _start
STDERR: