Relax assertion in ProxyConfigServiceLinux gconf cleanup.
Relax assertion in ProxyConfigServiceLinux gconf cleanup.

To wind down gconf, a cleanup task is posted to the UI thread. Don't
assert if that task is destroyed without being run (as may happen on
program exit).

BUG=16076 
TEST=Run "sh tools/valgrind/chrome_tests.sh -t ui", look af the first couple
     tests and check we don't get this assertion:
     "proxy/proxy_config_service_linux.cc(211)] Check failed: !client_."

Review URL: http://codereview.chromium.org/155363

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@20487 0039d316-1c4b-4281-b951-d872f2087c98

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