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For now this uses extensions_unittests_main.cc as there are no differences
in startup between extensions_unittests and app_shell_unittests.

This allows ShellNaClBrowserDelegate to have a test.

BUG=402612
TEST=app_shell_unittests

Review URL: https://codereview.chromium.org/471453003

Cr-Commit-Position: refs/heads/master@{#290719}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290719 0039d316-1c4b-4281-b951-d872f2087c98