Cleans up mistake from #2018439.

They methods are now safe to call repeatedly and will always produce the same
result. The value of #resizable is no longer overridden by Wysiwyg.

The current document should still be used as the context for fetching
a field by id, to ensure the parent form is still part of it.

Fixes regression introduced by #2284955.

- #507696 by hadsie, kndr, ksenzee, mcarbone, gaele, et al: Automatically adjust initial editor height.

Brandon Williams authored Feb 07, 2014 and Henrik Danielsson committed Feb 07, 2014

- #1956778 by mibfire, rocketeerbkw, guillaumev: Added support for configuring CKEditor's Advanced Content Filter.