[ios] Checks if PasswordDelegate is not nil when Modal was presented.

Since infobarModalPresentedFromBanner is called as completion
for the Modal presentation, there's a chance the Infobar could've been
destroyed by then, if this is the case passwordInfoBarDelegate was
set to nil in stop, and we shouldn't call InfobarPresenting.

Bug: 1055340
Change-Id: I603b7e6bdbd39094dc7cecea7f0e8084a91b6b24
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2072646Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744361}

ios/chrome/browser/ui/infobars/coordinators/infobar_password_coordinator.mm

@@ -177,7 +177,10 @@
   if (presentedFromBanner)
     return;
 
-  self.passwordInfoBarDelegate->InfobarPresenting(NO /*automatic*/);
+  // There's a chance the Delegate was destroyed while the presentation was
+  // taking place. Check if the delegate still exists.
+  if (self.passwordInfoBarDelegate)
+    self.passwordInfoBarDelegate->InfobarPresenting(NO /*automatic*/);
 }
 
 - (void)dismissBannerIfReady {