Move upload_file_system_file_element_reader.* to content/browser/fileapi/

BUG=338338
TBR=darin@chromium.org

Review URL: https://codereview.chromium.org/159523005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253037 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253037