Remove FileManager.filesystem_ field.

By this CL, DirectoryModel gets rid of "root" entry. So, we can remove
FileManager.filesystem_ field and its initialization code.
This is preparation to move the volumeManager to background page.

BUG=268233
TEST=Ran browser_tests --gtest_filter="*FileSystemExtensionApiTest*:*FileManagerBrowserTest*:*FileBrowserPrivateApiTest*" and tested manually.
R=isherman@chromium.org, yoshiki@chromium.org

Review URL: https://codereview.chromium.org/24538007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225680 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225680