Prevent autocomplete tests from being broken by data URL deprecation.

Content-initiated main-frame navigations to data URLs are being blocked
in crbug.com/594215. The tests fixed here were form-posting to themselves
which are data URLs and are going to be broken. Change form targets to
about:blank URLs instead.

BUG=594215

Review-Url: https://codereview.chromium.org/2797443004
Cr-Commit-Position: refs/heads/master@{#461786}
2 files changed