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Unit tests for both the PreviewsInfoBarDelegate and
PreviewsInfoBarTabHelper. Verfies that the infobar shows
with the correct strings and only shows once for a page.

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

Review-Url: https://codereview.chromium.org/2266653002
Cr-Commit-Position: refs/heads/master@{#418617}