Steps to repro with debug build:

1) Go to a YT page
2) Open the MR UI with cast button on the YT player and create a route.
3) Reopen the MR UI with the cast button on YT player - it should be displaying the custom controller with the route details.
4) Click "Close" to close the route and let the dialog autoclose.
5) Renderer will crash a few seconds later with the following stack trace:

ASSERTION FAILED: m_state == Detached || !m_isPromiseCalled || !getScriptState()->contextIsValid() || !getExecutionContext() || getExecutionContext()->activeDOMObjectsAreStopped()
../../third_party/WebKit/Source/bindings/core/v8/ScriptPromiseResolver.h(53) : virtual blink::ScriptPromiseResolver::~ScriptPromiseResolver()
1   0x7f902928f6ac
2   0x7f902928c621
3   0x7f902928c878
4   0x7f9029299bbb
5   0x7f9029299713
6   0x7f902928862e
7   0x7f9029299c91
8   0x7f902c003074
9   0x7f902c001c95
10  0x7f902a7a8aa9
11  0x7f902a739ce5
12  0x7f902a739ff8
13  0x7f902a73a52c
14  0x7f902a73babd
15  0x7f902a7397d1
16  0x7f902a75c810
17  0x7f902a738950
18  0x7f902a109f5c
19  0x7f902a2f2c3e
20  0x7f902a2f34e2
21  0x7f902a2f3f33
22  0x7f902a2f2800
23  0x7f90272c54db ChromeMain
24  0x7f9020202f45 __libc_start_main
25  0x7f90272c53b1
Received signal 11 SEGV_MAPERR 0000fbadbeef
#0 0x7f902a71ab07 base::debug::(anonymous namespace)::StackDumpSignalHandler()
#1 0x7f90230f7330 <unknown>
#2 0x7f90292d135d blink::ScriptPromiseResolver::~ScriptPromiseResolver()
#3 0x7f902928f6ac blink::NormalPage::sweep()
#4 0x7f902928c621 blink::BaseArena::sweepUnsweptPage()
#5 0x7f902928c878 blink::BaseArena::completeSweep()
#6 0x7f9029299bbb blink::ThreadState::eagerSweep()
#7 0x7f9029299713 blink::ThreadState::preSweep()
#8 0x7f902928862e blink::ThreadHeap::collectGarbage()
#9 0x7f9029299c91 blink::ThreadState::safePoint()
#10 0x7f902c003074 scheduler::TaskQueueManager::ProcessTaskFromWorkQueue()
#11 0x7f902c001c95 scheduler::TaskQueueManager::DoWork()
#12 0x7f902a7a8aa9 base::debug::TaskAnnotator::RunTask()
#13 0x7f902a739ce5 base::MessageLoop::RunTask()
#14 0x7f902a739ff8 base::MessageLoop::DeferOrRunPendingTask()
#15 0x7f902a73a52c base::MessageLoop::DoDelayedWork()
#16 0x7f902a73babd base::MessagePumpDefault::Run()
#17 0x7f902a7397d1 base::MessageLoop::RunHandler()
#18 0x7f902a75c810 base::RunLoop::Run()
#19 0x7f902a738950 base::MessageLoop::Run()
#20 0x7f902a109f5c content::RendererMain()
#21 0x7f902a2f2c3e content::RunZygote()
#22 0x7f902a2f34e2 content::RunNamedProcessTypeMain()
#23 0x7f902a2f3f33 content::ContentMainRunnerImpl::Run()
#24 0x7f902a2f2800 content::ContentMain()
#25 0x7f90272c54db ChromeMain
#26 0x7f9020202f45 __libc_start_main
#27 0x7f90272c53b1 <unknown>
  r8: 00007f9026766a00  r9: 00007f90272e52e0 r10: 00007f902059dbe0 r11: 0000000000000000
 r12: 00002a0a000df000 r13: 00002a0a000d7010 r14: 00002a0a000d7018 r15: 000014784b8a4010
  di: 0000000000000000  si: 0000000000000000  bp: 00000000000000c0  bx: 00002a0a000c0000
  dx: 0000000000000000  ax: 00000000fbadbeef  cx: 00007f902734aa40  sp: 00007ffdf9527bf0
  ip: 00007f90292d135d efl: 0000000000010246 cgf: 0000000000000033 erf: 0000000000000006
 trp: 000000000000000e msk: 0000000000000000 cr2: 00000000fbadbeef
[end of stack trace]


Observations:
- In step 3, if the UI is opened via browser menu or even in another tab, it still repros.
- Repros 100%.