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The welcome page was removed in M40. Users who had the welcome page open
when upgrading from pre-M40 to post-M40 will see a "webpage not
available" error after the upgrade. This fixes that behavior by
replacing the welcome page with an NTP.

BUG=455427

Review URL: https://codereview.chromium.org/1036533002

Cr-Commit-Position: refs/heads/master@{#322049}