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It was added in 2012 for PrintPreviewTest but is no longer needed and is
causing issues with single process mash tests, where OzonePlatform only
expects to get initialized once. If this happens in a test's SetUp(),
and parts of it get destroyed by the AtExitManager, they won't be re-
initialized for later tests. Tests will still pass in isolation but not
when run as a suite.

This CL also simplifies PrintPreviewTest a bit, which is the test fixture
for which the AtExitManager was originally added.

Bug: 905430
Change-Id: I4617aed098962f4eea6a9d1bfe052e3902278557
Reviewed-on: https://chromium-review.googlesource.com/c/1340798Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608948}