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The Lo-Fi snackbar has been replaced by the previews infobar.
Remove the snackbar and all associated code.

BUG=615566
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2263433002
Cr-Commit-Position: refs/heads/master@{#418595}