The preload webui screen lock code use IdleDetector -> UserActivityDetector, that is not created as part of brwoser. Hence the crash.
I will disable the preload for now and revisit this after issue 626899.
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Received signal 11 SEGV_MAPERR 000000000018
#0 0x7f9f0d1b36cb base::debug::StackTrace::StackTrace()
#1 0x7f9f0d1b1d9c base::debug::StackTrace::StackTrace()
#2 0x7f9f0d1b31df base::debug::(anonymous namespace)::StackDumpSignalHandler()
#3 0x7f9f0d5f7330 <unknown>
#4 0x7f9f0a400dec std::__cxx1998::vector<>::size()
#5 0x7f9f0a43fb58 base::ObserverListBase<>::HasObserver()
#6 0x7f9f0a43f18c ui::UserActivityDetector::HasObserver()
#7 0x7f9f0f05b19e chromeos::IdleDetector::Start()
#8 0x7f9f0f1a1b34 chromeos::PreloadedWebView::PreloadOnIdle()
#9 0x7f9f0f1052ba chromeos::WebUIScreenLocker::RequestPreload()
#10 0x7f9f0f14888a chromeos::ChromeSessionManager::SessionStarted()
#11 0x7f9f0f14b9f7 chromeos::UserSessionManager::DoBrowserLaunchInternal()
Comment 1 by bugdroid1@chromium.org
, Mar 7 2017