Explicitly cast istream to bool in file_system_dir_url_request_job_unittest.cc.

Implicit cast is forbidden in libc++.

BUG=318770
TBR=kinuko@chromium.org

Review URL: https://codereview.chromium.org/121823002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242637 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242637