Repro steps (may not be minimal):

0) Disable UIRefresh and SlimNavigationManager in about:flags.
1) Open two tabs and navigate each to any webpage.
2) Background the app, kill, and relaunch.
3) After relaunching, side-swipe to change tabs.

[0814/120500.130049:FATAL:web_state_impl.mm(146)] Check failed: observers_.HasObserver(observer). 
* thread #1, name = 'CrWebMain', queue = 'com.apple.main-thread', stop reason = EXC_BREAKPOINT (code=EXC_I386_BPT, subcode=0x0)
    frame #0: 0x00000001027fda04 Chromium`base::debug::BreakDebugger() at debugger_posix.cc:269
    frame #1: 0x000000010250fb7b Chromium`logging::LogMessage::~LogMessage(this=0x00007ffeef93d390) at logging.cc:865
    frame #2: 0x000000010250c495 Chromium`logging::LogMessage::~LogMessage(this=0x00007ffeef93d390) at logging.cc:593
    frame #3: 0x0000000101517985 Chromium`web::WebStateImpl::RemoveObserver(this=0x00007fe049e0ec60, observer=0x0000000000000000) at web_state_impl.mm:146
  * frame #4: 0x00000001020840b6 Chromium`::-[FormInputAccessoryMediator detachFromWebState](self=0x00006080000bcbc0, _cmd="detachFromWebState") at form_input_accessory_mediator.mm:133
    frame #5: 0x0000000102084e15 Chromium`::-[FormInputAccessoryMediator updateWithNewWebState:](self=0x00006080000bcbc0, _cmd="updateWithNewWebState:", webState=0x00007fe049d02fa0) at form_input_accessory_mediator.mm:195
    frame #6: 0x0000000102084dbe Chromium`::-[FormInputAccessoryMediator webStateList:didChangeActiveWebState:oldWebState:atIndex:reason:](self=0x00006080000bcbc0, _cmd="webStateList:didChangeActiveWebState:oldWebState:atIndex:reason:", webStateList=0x00006380000bc860, newWebState=0x00007fe049d02fa0, oldWebState=0x00007fe049e0ec60, atIndex=1, reason=2) at form_input_accessory_mediator.mm:186
    frame #7: 0x00000001019a63d6 Chromium`WebStateListObserverBridge::WebStateActivatedAt(this=0x000060800001e9c0, web_state_list=0x00006380000bc860, old_web_state=0x00007fe049e0ec60, new_web_state=0x00007fe049d02fa0, active_index=1, reason=2) at web_state_list_observer_bridge.mm:117
    frame #8: 0x000000010199cd9a Chromium`WebStateList::NotifyIfActiveWebStateChanged(this=0x00006380000bc860, old_web_state=0x00007fe049e0ec60, reason=2) at web_state_list.mm:342
    frame #9: 0x000000010199a6b2 Chromium`WebStateList::ActivateWebStateAt(this=0x00006380000bc860, index=1) at web_state_list.mm:315
    frame #10: 0x00000001023ebb47 Chromium`::-[TabModel setCurrentTab:](self=0x00007fe049e0c080, _cmd="setCurrentTab:", newTab=0x00006080000bae20) at tab_model.mm:292
    frame #11: 0x0000000101da553b Chromium`::__30-[CardSideSwipeView finishPan]_block_invoke.187(.block_descriptor=0x000063000024af20, finished=YES) at card_side_swipe_view.mm:483