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The test was disabled because ChromeOS will exit the test. But it is no
longer the case on ChromeOS. Let's enable it.

It is a bit weird to have this test inside
tab_contents_iterator_unittest.cc since it has little to do with tab
content. This CL moves the test to chrome/browser/lifetime/ since it
tests whether AttemptRestart() sets the preference correctly. We also
clean up the headers.

BUG=none

Change-Id: I663a0a7159b325b3783d816bec1d518ad7da7adb
Reviewed-on: https://chromium-review.googlesource.com/c/1352573
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611558}