Set SafeBrowsing download check to FULL_PING for unknown file types

This required updating several tests to use "safe" download types,
such as jpg or txt. For the download browser tests, the URL is forced
to be "download-known-size", so I changed the expectation to match
the format "Unconfirmed <number>.crdownload". A little bit of test data
needed to be changed, namely:

- simple_exe.cc was renamed simple_exe.txt, since cc is uncommon
- zipfile_no_binaries.zip now has contents "simple_exe.txt" instead of
  "simple_exe.cc", for the same reason.
- small_archive.rar now has contents "limerick.txt" instead of
  "limerick", since "limerick" is an uncommon extension.

Bug: 918991
Change-Id: Iaef193e32a825b3b977fdae55ec43e7c5aaa327f
Reviewed-on: https://chromium-review.googlesource.com/c/1459317
Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633800}
8 files changed