Add null check to prevent crash when saving password.

This check is needed since iOS Chrome is returning nullptr in GetPasswordAutofillManager()
https://cs.chromium.org/chromium/src/ios/chrome/browser/passwords/ios_chrome_password_manager_driver.mm

Bug: 890655
Change-Id: I33951b54d0e618f861284df7aeabcbf3d9e1681b
Reviewed-on: https://chromium-review.googlesource.com/1253164
Commit-Queue: Scott Wu <scottwu@chromium.org>
Reviewed-by: Vaclav Brozek <vabr@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595702}

components/password_manager/core/browser/password_manager.cc

@@ -591,8 +591,14 @@ void PasswordManager::UpdateFormManagers() {
 
   // The autofill manager will be repopulated again when the credentials
   // are retrieved.
-  for (PasswordManagerDriver* driver : drivers)
+  for (PasswordManagerDriver* driver : drivers) {
+    // GetPasswordAutofillManager() is returning nullptr in iOS Chrome, since
+    // PasswordAutofillManager is not instantiated on iOS Chrome.
+    // See //ios/chrome/browser/passwords/ios_chrome_password_manager_driver.mm
+    if (driver->GetPasswordAutofillManager()) {
       driver->GetPasswordAutofillManager()->DeleteFillData();
+    }
+  }
 }
 
 void PasswordManager::DropFormManagers() {