Steps:
1) run-webkit-tests -f

Expected: Does not crash in media/video-poster-load-after-playing.html layout test
Actual: Crashes in media/video-poster-load-after-playing.html

Crash log:

crash log for renderer (pid <unknown>):
STDOUT: layer at (0,0) size 800x600
STDOUT:   LayoutView at (0,0) size 800x600
STDOUT: layer at (0,0) size 800x124
STDOUT:   LayoutBlockFlow {HTML} at (0,0) size 800x124
STDOUT:     LayoutBlockFlow {BODY} at (8,8) size 784x108
STDOUT:       LayoutText {#text} at (0,0) size 0x0
STDOUT: layer at (8,8) size 76x103
STDOUT:   LayoutVideo {VIDEO} at (0,0) size 76x103
STDOUT: layer at (8,8) size 76x103
STDOUT:   LayoutFlexibleBox (relative positioned) {DIV} at (0,0) size 76x103
STDOUT:     LayoutBlockFlow {DIV} at (0,71) size 76x32
STDOUT: layer at (8,8) size 76x61
STDOUT:   LayoutFlexibleBox (relative positioned) {DIV} at (0,0) size 76x61
STDERR: [1:1:0720/204154:627716133159:WARNING:webmediaplayer_impl.cc(335)] Using MultibufferDataSource
STDERR: ASSERTION FAILED: m_state == Detached || !m_isPromiseCalled || !getScriptState()->contextIsValid() || !getExecutionContext() || getExecutionContext()->activeDOMObjectsAreStopped()
STDERR: ../../third_party/WebKit/Source/bindings/core/v8/ScriptPromiseResolver.h(53) : virtual blink::ScriptPromiseResolver::~ScriptPromiseResolver()
STDERR: 1   0x7f3c6bd493ec
STDERR: 2   0x7f3c6bd46511 blink::BaseArena::sweepUnsweptPage()
STDERR: 3   0x7f3c6bd46768 blink::BaseArena::completeSweep()
STDERR: 4   0x7f3c6bd53bfb blink::ThreadState::eagerSweep()
STDERR: 5   0x7f3c6bd53763 blink::ThreadState::preSweep()
STDERR: 6   0x7f3c6bd42233 blink::ThreadHeap::collectGarbage(blink::BlinkGC::StackState, blink::BlinkGC::GCType, blink::BlinkGC::GCReason)
STDERR: 7   0x7f3c6bd4fe82 blink::ThreadState::performIdleGC(double)
STDERR: 8   0x7f3c6bb24e23
STDERR: 9   0x7f3c688b5ad1 scheduler::WebSchedulerImpl::runIdleTask(std::unique_ptr<blink::WebThread::IdleTask, std::default_delete<blink::WebThread::IdleTask> >, base::TimeTicks)
STDERR: 10  0x7f3c688b5f94
STDERR: 11  0x7f3c688b52ca
STDERR: 12  0x7f3c688b56af
STDERR: 13  0x7f3c6e460479 base::debug::TaskAnnotator::RunTask(char const*, base::PendingTask const&)
STDERR: 14  0x7f3c688a9297 scheduler::TaskQueueManager::ProcessTaskFromWorkQueue(scheduler::internal::WorkQueue*, scheduler::internal::TaskQueueImpl::Task*)
STDERR: 15  0x7f3c688a7f29 scheduler::TaskQueueManager::DoWork(base::TimeTicks, bool)
STDERR: 16  0x7f3c688aa1a9
STDERR: 17  0x7f3c6e460479 base::debug::TaskAnnotator::RunTask(char const*, base::PendingTask const&)
STDERR: 18  0x7f3c6e48ad55 base::MessageLoop::RunTask(base::PendingTask const&)
STDERR: 19  0x7f3c6e48b088 base::MessageLoop::DeferOrRunPendingTask(base::PendingTask)
STDERR: 20  0x7f3c6e48b43b base::MessageLoop::DoWork()
STDERR: 21  0x7f3c6e48cbfe base::MessagePumpDefault::Run(base::MessagePump::Delegate*)
STDERR: 22  0x7f3c6e48a851 base::MessageLoop::RunHandler()
STDERR: 23  0x7f3c6e4b9060 base::RunLoop::Run()
STDERR: 24  0x7f3c6f65712a
STDERR: 25  0x7f3c6f76efbb
STDERR: 26  0x7f3c6f76f862
STDERR: 27  0x7f3c6f7702b3
STDERR: 28  0x7f3c6f76eb80 content::ContentMain(content::ContentMainParams const&)
STDERR: 29  0x49d47b
STDERR: 30  0x7f3c65fcef45 __libc_start_main
STDERR: 31  0x49d371

This happened during GC, so it may be hard to reproduce.

ToT was f7dbf39be31d8aa9214d5d84da613508d4e06491 (July 14th 2016).