Steps to reproduce: 
Open Clean Skeleton App
Open New Tab
Navigate to a web page.
Long press on back button.

Observed behavior: 
Crash.

Expected behavior: 
Backward history menu opens.


Additional comments: 
The cause is that the HistoryPopupCoordinator does not implement a viewController property, as required by the BrowserCoordinator API. This property is accessed in BrowserCoordinator's updateActivationStateAfterTransition method.

Note also that the CHECK that was intended to catch this case still passed: https://cs.chromium.org/chromium/src/ios/chrome/browser/ui/coordinators/browser_coordinator.mm?q=browsercoordinator&dr=C&l=156

Here is the stack trace:
2017-09-19 18:38:04.134 CSChromium[75095:81600699] -[HistoryPopupCoordinator viewController]: unrecognized selector sent to instance 0x6000000fed00

2017-09-19 18:38:04.152 CSChromium[75095:81600699] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[HistoryPopupCoordinator viewController]: unrecognized selector sent to instance 0x6000000fed00'

*** First throw call stack:

(

0   CoreFoundation                      0x000000011bfd234b __exceptionPreprocess + 171

1   libobjc.A.dylib                     0x00000001203ff21e objc_exception_throw + 48

2   CoreFoundation                      0x000000011c041f34 -[NSObject(NSObject) doesNotRecognizeSelector:] + 132

3   CoreFoundation                      0x000000011bf57c15 ___forwarding___ + 1013

4   CoreFoundation                      0x000000011bf57798 _CF_forwarding_prep_0 + 120

5   CSChromium                          0x000000010ed3ee6c -[BrowserCoordinator updateActivationStateAfterTransition] + 348

6   CSChromium                          0x000000010ed3e97d -[BrowserCoordinator start] + 189

7   CSChromium                          0x000000011048dca1 -[HistoryPopupCoordinator start] + 1409

8   CSChromium                          0x000000011047881c -[ToolbarCoordinator showTabHistoryPopupForBackwardHistory] + 1804

9   CSChromium                          0x000000011048023b -[ToolbarViewController handleLongPress:] + 299

10  UIKit                               0x000000011abd5289 -[UIGestureRecognizerTarget _sendActionWithGestureRecognizer:] + 57

11  UIKit                               0x000000011abdd028 _UIGestureRecognizerSendTargetActions + 109

12  UIKit                               0x000000011abdaaf7 _UIGestureRecognizerSendActions + 227

13  UIKit                               0x000000011abd9d83 -[UIGestureRecognizer _updateGestureWithEvent:buttonEvent:] + 891

14  UIKit                               0x000000011abc5e56 _UIGestureEnvironmentUpdate + 1395

15  CoreFoundation                      0x000000011bf76e17 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 23

16  CoreFoundation                      0x000000011bf76d87 __CFRunLoopDoObservers + 391

17  CoreFoundation                      0x000000011bf5bb9e __CFRunLoopRun + 1198

18  CoreFoundation                      0x000000011bf5b494 CFRunLoopRunSpecific + 420

19  GraphicsServices                    0x0000000121604a6f GSEventRunModal + 161

20  UIKit                               0x000000011a692f34 UIApplicationMain + 159

21  CSChromium                          0x000000010ed38e89 main + 937

22  libdyld.dylib                       0x0000000123af068d start + 1

)

libc++abi.dylib: terminating with uncaught exception of type NSException