Remove ash/system references from WebUILoginView

Bug: 854346
Test: the new test case
Change-Id: I5a78d4037ffcf2daf995c2fc04ef44459abbd21c
Reviewed-on: https://chromium-review.googlesource.com/c/1338187
Commit-Queue: Jun Mukai <mukai@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612451}

• ash/BUILD.gn[diff]
• ash/login/login_screen_controller.cc[diff]
• ash/login/login_screen_controller.h[diff]
• ash/login/login_screen_controller_unittest.cc[diff]
• ash/login/mock_login_screen_client.h[diff]
• ash/login/ui/lock_contents_view.h[diff]
• ash/public/cpp/BUILD.gn[diff]
• ash/public/cpp/system_tray_focus_observer.h[Added - diff]
• ash/public/interfaces/login_screen.mojom[diff]
• ash/shell.cc[diff]
• ash/system/status_area_widget_unittest.cc[diff]
• ash/system/system_tray_focus_observer.h[Deleted - diff]
• ash/system/tray/system_tray_notifier.cc[diff]
• chrome/browser/chromeos/login/ui/DEPS[diff]
• chrome/browser/chromeos/login/ui/webui_login_view.cc[diff]
• chrome/browser/chromeos/login/ui/webui_login_view.h[diff]
• chrome/browser/ui/ash/login_screen_client.cc[diff]
• chrome/browser/ui/ash/login_screen_client.h[diff]