Fix crash occurring during infobar dismissal at BVC deletion.

Calling the InfoBarContainerIOS destructor leads to the removal of infobars via
|InfoBarContainerIOS::PlatformSpecificRemoveInfoBar|, which leads to the BVC
wanting to change the InfoBarContainerIOS's height (in |infoBarContainerStateChanged|)
This leads to a crash because the InfoBarContainerIOS is in the process of
being deleted.

This CL makes the flow abort early.

BUG=448093

Review URL: https://codereview.chromium.org/1038903003

Cr-Commit-Position: refs/heads/master@{#322432}

ios/chrome/browser/infobars/infobar_container_ios.mm

@@ -22,6 +22,7 @@ InfoBarContainerIOS::InfoBarContainerIOS(
 }
 
 InfoBarContainerIOS::~InfoBarContainerIOS() {
+  delegate_ = nullptr;
   RemoveAllInfoBarsForDestruction();
 }
 
@@ -44,7 +45,7 @@ void InfoBarContainerIOS::PlatformSpecificRemoveInfoBar(
   // Otherwise, the infobar is being replaced by another one. Do not call the
   // delegate in this case, as the delegate will be updated when the new infobar
   // is added.
-  if (infobar->total_height() == 0)
+  if (infobar->total_height() == 0 && delegate_)
     delegate_->InfoBarContainerStateChanged(false);
 
   // TODO(rohitrao, jif): [Merge 239355] Upstream InfoBarContainer deletes the