Remove unnecessary sandbox_file_system_backend.h dependency

BUG=265769
TEST=build

Review URL: https://codereview.chromium.org/26479002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227682 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227682