FileSystem: Move SnapshotFileCallback into FileSystemCallbacks.h

This move also fixes the bug for FileEntry.file() operation.

BUG=370100
R=kinuko@chromium.org, nhiroki@chromium.org, yurys

Review URL: https://codereview.chromium.org/388923004

git-svn-id: svn://svn.chromium.org/blink/trunk@178037 bbb929c8-8fbe-4397-9dbb-9b2b20218538