Create Gaia sign webview in chrome for views login screen.

This is a basic flow of adding a new user in the views login screen.
Changes including:
1. Add ShowGaiaSignin mojo call for the views login screen to request
show gaia screen in chrome.
2. Create GaiaDialogDelegate to manage the behavior of the dialog which
hosts the gaia content.
3. Updating LoginHostDisplay to support views login case:
Act like an intermediate layer for communication between GaiaScreenHandler
and GaiaDialogDelegate. Also owns the WizardController to support post
login screens.
4. Add handler functions in GaiaScreenHandler to update the gaia screen
size and visibility. Move HandleShowAddUser/OnShowAddUser from
SigninScreenHandler to GaiaScreenHandler because its more relevant to
gaia screen.
5. Update JS code to handle views login case.


Bug: 792579
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I2802e857eb3c6c825e0a463010c856958badd2b7
Reviewed-on: https://chromium-review.googlesource.com/892042
Commit-Queue: Xiaoyin Hu <xiaoyinh@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534867}

• ash/login/login_screen_controller.cc[diff]
• ash/login/login_screen_controller.h[diff]
• ash/login/mock_login_screen_client.h[diff]
• ash/public/interfaces/login_screen.mojom[diff]
• ash/shelf/login_shelf_view.cc[diff]
• chrome/browser/chromeos/BUILD.gn[diff]
• chrome/browser/chromeos/arc/arc_session_manager_unittest.cc[diff]
• chrome/browser/chromeos/login/screens/gaia_view.h[diff]
• chrome/browser/chromeos/login/ui/gaia_dialog_delegate.cc[Added - diff]
• chrome/browser/chromeos/login/ui/gaia_dialog_delegate.h[Added - diff]
• chrome/browser/chromeos/login/ui/login_display_host.h[diff]
• chrome/browser/chromeos/login/ui/login_display_host_views.cc[diff]
• chrome/browser/chromeos/login/ui/login_display_host_views.h[diff]
• chrome/browser/chromeos/login/ui/login_display_host_webui.cc[diff]
• chrome/browser/chromeos/login/ui/login_display_host_webui.h[diff]
• chrome/browser/chromeos/login/ui/login_display_views.cc[diff]
• chrome/browser/chromeos/login/ui/mock_login_display_host.h[diff]
• chrome/browser/resources/chromeos/login/md_login.js[diff]
• chrome/browser/resources/chromeos/login/screen_gaia_signin.js[diff]
• chrome/browser/ui/ash/login_screen_client.cc[diff]
• chrome/browser/ui/ash/login_screen_client.h[diff]
• chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.cc[diff]
• chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.h[diff]
• chrome/browser/ui/webui/chromeos/login/oobe_ui.cc[diff]
• chrome/browser/ui/webui/chromeos/login/oobe_ui.h[diff]
• chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc[diff]
• chrome/browser/ui/webui/chromeos/login/signin_screen_handler.h[diff]
• ui/login/display_manager.js[diff]