Use factory methods of FileEntrySync and DirectoryEntrySync in EntrySync::create.

Also, stop their friendship.

BUG=none
TEST=none; no behaivor changes.

Review URL: https://codereview.chromium.org/131103016

git-svn-id: svn://svn.chromium.org/blink/trunk@165602 bbb929c8-8fbe-4397-9dbb-9b2b20218538