Use null for uninitialized DirectoryContent's current directory entry.

Because Files.app will support multi-filesystem, '/' shouldn't be resolved.
This is the preparation to remove the dependency to '/' entry.

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

Review URL: https://codereview.chromium.org/24745002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225671 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225671