cros: Propagate parent window for GetHelp App on the lock screen

Fixed: 1022774
Change-Id: I2ffc80512230316cc3b94b70e7e11f6b2db0390e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2010950
Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734377}

• ash/login/login_screen_controller.cc[diff]
• ash/login/login_screen_controller.h[diff]
• ash/login/mock_login_screen_client.h[diff]
• ash/login/ui/lock_contents_view.cc[diff]
• ash/login/ui/lock_contents_view_unittest.cc[diff]
• ash/login/ui/parent_access_view.cc[diff]
• ash/login/ui/parent_access_view_unittest.cc[diff]
• ash/public/cpp/login_screen_client.h[diff]
• chrome/browser/chromeos/login/ui/login_web_dialog.cc[diff]
• chrome/browser/ui/ash/login_screen_client.cc[diff]
• chrome/browser/ui/ash/login_screen_client.h[diff]