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Some of the tests run by filename_util_unittests invoke
GenerateFilename() which in turn consults the host platform's MIME type
to file extension map when determining a filename. This causes tests to
fail on systems where the file extension for image/jpeg was something
other than .jpg.

This CL removes the MIME type dependence from tests that shouldn't be
exercising it. It also changes the MIME type from image/jpeg to
application/x-chrome-extension for the few remaining tests that do. The
latter shouldn't be set on any platform. In addition, a new tests
correctly flags the underlying cause if tests were to be run on such a
system.

Bug: 828038
Change-Id: I7e2ea4c6147c0a8de72e4d1b5dbf46ad286a4599
Reviewed-on: https://chromium-review.googlesource.com/990998
Commit-Queue: Asanka Herath <asanka@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547706}