drive: Move ClientContext from file_system_interface.h to job_scheduler.h

FileSystemInterface has nothing to do with ClientContext

BUG=None
TEST=build
R=kinaba@chromium.org

Review URL: https://codereview.chromium.org/168363003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252188 0039d316-1c4b-4281-b951-d872f2087c98
LKGR w/ DEPS up to revision 252188