Implement x-button for webview based signin
Update assets for x-button.
Added 3 states for the button: on blue background (normal),
on white background (loading) and masked (modal dialog shown).
Hide invisible but working by mistake x-button for normal flow.
BUG=
463511
TEST=manual
Committed:
https://crrev.com/4da1323a144eab21b1dbd287a24f254e34619be1
Cr-Commit-Position: refs/heads/master@{#320473}