Don't pass in the parent window for the parent access code help dialog

When the parent access code feature first launched, there was no parent
passed in, and this regression was introduced in
https://crrev.com/c/2010950.
The parent window for this particular help dialog is small, and it is ok
for the help dialog fall back on the default size for dialogs without a
parent.

Bug: 1128724
Change-Id: I7fc75cb66ea51d4208065bd4e75da72843e25aab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2982647
Reviewed-by: Aga Wronska <agawronska@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Courtney Wong <courtneywong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#897559}

• ash/child_accounts/parent_access_controller_impl.cc[diff]
• ash/child_accounts/parent_access_controller_impl.h[diff]
• ash/child_accounts/parent_access_controller_impl_unittest.cc[diff]
• ash/login/login_screen_controller.cc[diff]
• ash/login/login_screen_controller.h[diff]
• ash/login/mock_login_screen_client.h[diff]
• ash/login/security_token_request_controller.cc[diff]
• ash/login/security_token_request_controller.h[diff]
• ash/login/ui/pin_request_view.cc[diff]
• ash/login/ui/pin_request_view.h[diff]
• ash/login/ui/pin_request_view_unittest.cc[diff]
• ash/public/cpp/login_screen_client.h[diff]
• chrome/browser/ui/ash/login_screen_client_impl.cc[diff]
• chrome/browser/ui/ash/login_screen_client_impl.h[diff]